The fabrication of cermets is a cumbersome task faced by both scholars and engineers. The synthesis route of cermets plays a predominant role in influencing the properties of the cermet system. There are limitations associated with each method used, and researchers are still seeking to identify the best possible means of producing cermets with the desired properties. The processing parameters significantly affect the final property and microstructure of the cermet system. This suggests the need to choose the appropriate synthesis route. The following section discusses various conventional and modern techniques used to fabricate cermets. There is constant development of new fabrication techniques, but, within the scope of this paper, only a few selected methods, for which a considerable amount of study on the cermet fabrication process has been undertaken, are reviewed. This section aims to provide the reader with an understanding of previous and existing techniques in cermet synthesis, the effect of process parameters, and the advantages and disadvantages associated with each method described.
The fabrication of cermets is a cumbersome task faced by both scholars and engineers. The synthesis route of cermets plays a predominant role in influencing the properties of the cermet system. There are limitations associated with each method used, and researchers are still seeking to identify the best possible means of producing cermets with the desired properties. The processing parameters significantly affect the final property and microstructure of the cermet system. This suggests the need to choose the appropriate synthesis route. The following section discusses various conventional and modern techniques used to fabricate cermets. There is constant development of new fabrication techniques, but, within the scope of this paper, only a few selected methods, for which a considerable amount of study on the cermet fabrication process has been undertaken, are reviewed. This section aims to provide the reader with an understanding of previous and existing techniques in cermet synthesis, the effect of process parameters, and the advantages and disadvantages associated with each method described.
The polymer-derived ceramic (PDC) composites are an interesting category of materials with unique properties compared to conventional ceramic materials. The transformation of polymer to ceramic helps in achieving significant breakthroughs in ceramic technology. The composites find applications in wide areas, including aerospace, nuclear, and defense industries, where the oxidation resistance and high-temperature properties of PDC are beneficial [ 74 76 ]. The capability to fabricate complex shapes with the PDC enhances the application areas for this processing route. The advancements in this area have been in obtaining PDC as coatings, fibers, reinforcements, or ceramics which are stable at ultra-high temperatures (up to °C). However, the synthesis of cermets through the PDC route is viable and can be further investigated in the future.
Liquid-phase sintering is effective when there is good wettability and solubility of the hard phase in the liquid. The absence of such a mechanism in most cermet systems requires other processes, such as pressure-assisted sintering (hot uniaxial or isostatic pressing) to obtain pore-less materials in non-wetting systems, such as oxide cermets. These techniques are carried out at relatively low temperatures to eliminate the formation of undesirable phases. Spark plasma sintering is a recent development for sintering at relatively low temperatures within a short period. The infiltration of molten metal into the ceramic is another development in the synthesis of cermet systems. This method overcomes the limitation of the excessive shrinkage associated with liquid-phase sintering and produces accurate dimensions in complex shapes. The infiltration system is most effective with a wetting system where it penetrates the matrix by capillary action. A pressure-assisted infiltration system is employed for metal matrix composite fabrication, and can be utilized in the fabrication of cermets, but the high-volume content of ceramic in the matrix makes this process difficult. The infiltration technique can also be applied to fabricate functionally graded cermets.
In most cases, the cermets possess a typical core-shell morphology. The core consists of the undissolved particles, whereas the shell is produced by sintering through reaction and precipitation. The core and shell possess identical crystal orientations. The ceramic particles join each other and form a continuous network during the sintering process to minimize surface energy. A similar interconnected network formation also occurs with metallic binders. The metallic binders in this constrained state exhibit features different from those of the free metal [ 73 ]. The distribution of the network of metallic binders and the formation and characteristics of ceramic particles within the cermet system greatly influence the final properties of the system.
In many cases, an additional shaping operation is required to obtain the final shape. The cold compacted product is then sintered with fully dense cermets obtained using a high-temperature PM process. This is generally performed in ovens under an inert atmosphere, vacuum, or in hydrogen. Depending on the ceramic-metal system, liquid or solid-phase sintering is performed. The liquid-phase sintering enhances densification by rearranging particles, grain coarsening, and solution precipitation [ 72 ]. Following the sintering process, hot isostatic pressing (HIP) may be carried out to eliminate residual porosities. The solid-state sintering process achieves bonding and densification by applying heat below the melting point of the materials. Solid-state sintering processes, such as powder-rolling and warm extrusion, are utilized to produce wires or slabs. Figure 2 depicts the steps involved in the PM processing of cermets. The sintering process leads to building up of the microstructure, morphology, and phases of the cermet.
The most common methods of cermet fabrication involve PM techniques. The main steps involved in PM processes are the mixing of powder, milling of the powder mix to obtain proper intermixing and the desired particle size, compaction, and sintering. The powders of ceramic and metal particles are thoroughly mixed and further milled in ball mills or attritors. To prevent the agglomeration of the powder and oxidation, a wetting organic lubricant, such as polyglycol or paraffin wax, is added along with the mixture. The milling process facilitates the uniform distribution of ceramic grain sizes and embeds the ceramic particles with metal, which aids in the sintering process. The formed slurry is dried to expel the solvent, and, eventually, the powder forms as spherical granules with dimensions in the range of 0.1 mm&#;0.5 mm [ 5 ]. The milling process is followed by the powder compaction step. The part is formed into the required shape during this step. Cold pressing is the most commonly used method for relatively simpler shapes. Complex shapes can be achieved by PM injection molding, cold isostatic pressing, or extrusion slip casting.
Single and multiphase materials with oxide and non-oxide ceramics, intermetallics, and metals can be synthesized using RHP. The powder preformed in the porous starting condition is transformed into desirable phases involving reactions such as reduction reactions, displacement reactions, and elemental precursors reactions [ 81 ]. Cermets that include Al/Ni and Al/Nb are synthesized with this method. LaSalvia et al. [ 82 ] have successfully synthesized TiC-30% Ni and studied the effect of varying the Mo addition on its microstructural and mechanical properties.
The reaction synthesis process, owing to its ability to produce CMCs with desired microstructural features and tailored properties, has recently attracted much attention as a fabrication technique for CMCs. The various reaction synthesis processes, including directed metal oxidation (DMO), reactive metal penetration (RMP), reaction bonding (RB), reactive hot pressing (RHP) and reactive forging (RF) can be used to fabricate cermets with a typical three-dimensionally interconnected ceramic reaction product with some metallic content. An alumina ceramic matrix with residual Al metal can be achieved with the DMO approach. The DMO process has been implemented successfully for several cermet systems, including carbides, nitrides, borides, and oxides of Al, Ti, Zr, Hf, and Si [ 77 80 ].
Wayne et al. [ 97 ] analyzed the microstructure, mechanical, and wear properties of thermally sprayed and sintered WC-Co cermet systems with two different types of binder content. The influence of cobalt content was not significant in the wear resistance to diamond abrasion of the thermally sprayed cermet. However, it showed a greater effect in controlling the wear resistance due to particle erosion. The abrasion wear and erosion wear resistance of the studied cermet system mainly depended on the porosity, mean free path, and carbide grain size of the binder. It was evident from the observations that, for coatings, the porous structure resulted in poor intersplat bonds, which further reduced the hardness and fracture toughness. These observations are in contrast to those of other researchers because of the difference in the tribological conditions used.
The spraying of WC-based cermet coatings is one of the most significant applications of the HVOF method because of the reduced carbide transformation due to the low temperature received by the particles. High particle velocities are attained using a converging-diverging de Laval nozzle design and high gas pressures. Solid particle erosion (SPE) plays a critical role in the degradation of materials. Cermet systems based on WC and chromium carbide (CrC) coated by HVOF have shown impressive erosion behavior [ 93 94 ]. Kumar et al. [ 95 ] experimented on WC and CrC-based HVOF thermally sprayed cermet coatings to study their erosion behavior and investigated their effectiveness in the application of pulverized coal burner nozzles (PCBN) to mitigate SPE. It was observed that the coatings failed to sustain the erosion attacks and exposed the substrate. One possible reason for this was the larger particle velocity and the flux incorporated to mimic the PCBN conditions. It is still possible for the coatings to sustain particles moving in low velocities but they fail to do so in applications involving high velocity together with high particle flux. Wood et al. [ 96 ] investigated the tribology of thermally sprayed WC-Co coatings. They observed a more than 50% improvement in erosion resistance for HVOF coatings after parameter optimization compared to coating achieved with a conventional D-gun with similar nominal composition. The abrasive resistance of these coatings was comparable to sintered cermets having the same composition. The coating was found to have relatively high dry sliding and wet sliding wear resistance with a COF ranging from 0.2 to 0.5. The friction and wear appeared to be influenced by the oxide presence on the binder.
HVOF is a TS coating technique that can produce coatings with a very dense and compact structure and good adhesion to the substrate. Recent advancements in HVOF spraying have made it possible to produce coatings with lower porosities and decarburization [ 91 ]. The HVOF technique produces an efficient deposit of composite coatings with higher density, good bond strength, and comparatively lower decarburization. This is because of the high particle velocities and relatively lower temperature during deposition. In the HVOF process, the fuel and oxygen are intermixed and burnt in the combustion chamber at higher flow rates of up to L/min, and with a pressure range of up to 12 bars. This produces a high-speed gas jet. Powder particles with a size in the range of 5&#;70 µm are injected into the high-speed gas jet and accelerated towards the substrate. The heated powders are deposited on the substrate at 600&#;650 m/s. Upon impact, they form lenticular splats and adhere strongly to each other and the substrate. Raster movement of the HVOF gun in several passes achieves the required coating thickness [ 92 ].
The TS coating is performed by melting materials, which are in wire stock or particulate form, and accelerating the partially melted or fully melted particle droplets towards a substrate. As the droplets strike the substrate, they expand out radially to form a &#;splat&#;. With continued deposition of these splats, they eventually interact and combine to form a continuous coating. High particle velocities or high temperature during the impact can improve the bonding between these splats and the removal of pores. Various processes have varying thermal and kinetic energy contents [ 88 ]. TS can produce metallic, ceramic, carbide, and cermet coatings with any phase composition on a properly prepared base. In general, TS coatings possess good adhesion strength with various substrates. They have good wear resistance and low corrosion rates. Typically, these are applied to weaker underlying materials to ensure improved wear properties, thereby increasing the life of the components [ 89 ].
Thermal spray (TS) processes are growing rapidly and represent an important surface modification technology [ 83 ]. They have become an important modern industrial tool capable of producing customized surface properties for a range of industrial applications, which include thermal barrier coatings for turbine blades, erosion-resistant coatings for boiler tubes, and so on [ 84 ]. Like most coating techniques, TS coating helps in combining the advantages of the core with increased hardness, resistance to abrasive wear, and heat resistance [ 85 ]. TS is one of the most frequently used protective coating technologies. The TS technique is carried out by partially or completely melting the spraying material for milliseconds, then propelling it onto the surface to be coated with highly accelerated velocities [ 86 ]. The microstructure and properties of the thermally sprayed components depend on the powder characteristics and processing parameters [ 87 ].
Cold spraying (CS) or cold gas dynamic spraying is a process where solid powders are deposited on a substrate using a de Laval nozzle. To explore specific properties, CS can effectively deposit various materials, including metals, ceramics, polymers, and composites. As the particle&#;s impact velocity exceeds a threshold limit, it undergoes plastic deformation and adheres to the substrate [ 98 ]. Because of the low gas temperatures arising due to the rapid gas expansion in the nozzle, the feedstock powders stay solid throughout the complete travel of the nozzle [ 71 ]. The CS technique can form very dense coatings with very low oxygen content without grain growth, residual tensile stresses, or phase changes. The deposition of some materials can even produce grain refinement at the nanoscale. These properties make CS suitable for the deposition of a set of advanced materials. This is an efficient and novel technique to produce surface coatings with several advantages over TS. It uses the particle&#;s kinetic energy instead of thermal energy for deposition. Through this, oxidation, undesired chemical reactions, and tensile residual stress can be eliminated [ 99 ]. The low temperatures in CS lead to the occurrence of unique characteristics. The CS technique can retain the microstructure and properties of the feedstock powders and avoid oxide formation and undesirable structural changes. This helps in improving the durability of the coating. CS coatings are used to deposit not only metals, as was its initial purpose, but also polymers, ceramics, and advanced composites. As the particles contact the substrate, they experience severe plastic deformation due to the kinetic energy released. They are adhered to the substrate by mechanical anchoring. If there is sufficient plastic deformation, metallurgical bonding facilitates adherence [ 98 101 ]. In the conventional case of cold spraying of metals on metallic substrates, the adhesion between the substrate and the metal occurs due to the combined effect of metallurgical bonding and interlocking via the adiabatic shear instability (ASI) mechanism [ 98 ].
There are two types of CS systems: high-pressure cold spray (HPCS) and low-pressure cold spray (LPCS) systems. In the HPCS system, particles are injected ahead of the nozzle throat from a high-pressure gas supply. In contrast, in the LPCS system, the powder particles are injected into the diverging section of the nozzle by a low-pressure gas supply. In high-pressure cold spraying ( Figure 3 ), a preheated high-pressure gas (up to psi), either nitrogen or helium, is forced through a converging-diverging de Laval nozzle. The nozzle converts enthalpy into kinetic energy by expansion and accelerates the gas flow to a supersonic region ( m/s), causing a temperature reduction. The powder is supplied axially into the gas stream before reaching the nozzle throat. The gas carries the powder particles with sufficient acceleration and impacts the substrate. The particle&#;s kinetic energy is sufficient to induce a mechanical or metallurgical bond between particles and the substrate [ 99 102 ].
In low-pressure cold spraying ( Figure 4 ), air or nitrogen is used as the carrier gas with relatively low-pressure values (80&#;140 psi). The preheated gas is forced through a de Laval nozzle and accelerated to 600 m/s. Then, the powder is supplied into the nozzle&#;s diverging section (downstream) and accelerated towards the substrate [ 103 ].
104,105,The cold spraying of cermets is performed by mixing the reinforcement particles with the ductile metallic feedstock powders. Materials such as ceramics, oxides, and so on, in their pure form, cannot produce a coating on any surface without causing surface erosion. However, numerous studies have reported the possibility of depositing already prepared cermet mixtures on different substrates [ 68 106 ]. By this method, metal coatings with ceramic inclusions can be produced. Only a limited amount of the reinforcement particles are retained in the final coating. The process does not induce severe plastic deformation to the hard reinforcement particles as is the case with ductile materials, but rather leads to more plastic deformation in ductile materials. These particles embed themselves in the coating, producing cermets [ 68 70 ].
107,Several ceramic-metal coating combinations have been successfully obtained using the CS technique [ 70 108 ]. The intended properties and applications of these coatings are almost the same as in the case of TS techniques. These mainly include wear resistance, high hardness, and high-temperature hardness. The addition of ceramic particles is the main reason for enhancing these properties. It also influences the deposition process behavior. There are three main theories that have been proposed to explain the influence of ceramic particles on the coating and deposition properties. One proposed mechanism involves the peening or impingement effect of ceramic particles [ 109 ]. It is proposed that, as the spraying happens, the hard ceramic particles behave like shot balls, which peen the softer particles in front of them, causing more deformation. This, in turn, increases the DE of softer metallic particles. This mechanism has also been described in several other studies [ 107 110 ].
Irissou et al. [ 70 111 ] proposed another mechanism for the improved DE of the CS technique. They observed that as the ceramic content in the feedstock powder mixture increases, the roughness of the interface between the coating and the interface increases. The increase in roughness is attributed to the grit blasting action of the impacting ceramic particles. In addition, they suggest that the asperities formed during this process help to bond more particles, thereby improving the DE due to mechanical anchoring. The third mechanism suggested is related to the oxide cleaning action of ceramic particles on metallic particles or substrates. The ceramic particles, upon impacting the metallic particles or substrates, remove the oxide films, thereby exposing the fresh surface of the material for the further impact of metallic particles. This creates favorable bonding sites. The same effect can be achieved by the pure deformation of the brittle oxide layer, shredding from the surface, and improving the DE [ 112 113 ]. The optimization of parameters serves a crucial role in producing quality CS materials. Couto et al. [ 114 ] experimented with studying the wear and corrosion properties of WC-based cermets with two distinct binder proportions (WC-7Co and WC-12Co) coated on an Al -T6 aluminum substrate using the CS technique. They optimized the critical parameters influencing the properties of the deposited coating, including the temperature, spraying angle, spraying distance, gas pressure, and gas medium. They were successful in producing a dense and well-bonded coating on the substrate. Significant features of the processing route included that the coating obtained after spraying had no microstructural changes, decarburization, or any unwanted phase formation, indicating that the bulk properties of the feedstock powder were preserved. It was observed that higher temperature resulted in denser and thicker coatings on both occasions for the WC-Co cermet.
2O3 and SiC). This also led to an increase in the DE of the metal component of the mixture compared to the deposition of pure metal alone. The coarser ceramic powders negatively influenced the process by creating a strong erosion effect, which considerably reduced the deposition efficiency of the metal over the substrate. The deposition behavior and DE for several Al-Al2O3 cermet systems were investigated by Fernandez et al. [2O3 feedstock powders to analyze the deposition behavior and influence of the ceramic content on deposition efficiency. An increase in DE was observed, with a peak value obtained with 30 wt.% of ceramic content, followed by a gradual decrease, and eventually no deposition at 100% Al2O3. The experiment demonstrated the positive impact of the presence of ceramic particles in improving the DE of Al coating.Dosta et al. [ 115 ] ] conducted a similar study of the wear and corrosion resistance of WC-25Co cermet cold sprayed on carbon steel and Al-T6. They measured the bonding strength of the coating by adhesion testing based on ASTM C633-08. Sliding and abrasive resistance were also measured. The main objective of this study was to optimize the spraying conditions of the cermet system and the CS system to obtain good quality coatings. Dense and thick coatings on both substrates were obtained using the CS technique. The coating obtained had no decarburization, microstructural change, or unwanted phases. The CS technique is promising and can produce better coatings than the TS coating technique. A major concern associated with ceramic particles in the CS technique is the erosion that might occur to the metallic particles or substrate. The influence of particle size on the properties of cermet coatings was studied experimentally by Sova et al. [ 68 ]. They experimented with a specially developed nozzle with a separate metal powder and ceramic injection into the gas stream. They observed a strong activation effect and better coating for spraying soft metals (Al, Cu) while incorporating them with fine ceramic powders (Aland SiC). This also led to an increase in the DE of the metal component of the mixture compared to the deposition of pure metal alone. The coarser ceramic powders negatively influenced the process by creating a strong erosion effect, which considerably reduced the deposition efficiency of the metal over the substrate. The deposition behavior and DE for several Al-Alcermet systems were investigated by Fernandez et al. [ 71 ]. They used different Al-Alfeedstock powders to analyze the deposition behavior and influence of the ceramic content on deposition efficiency. An increase in DE was observed, with a peak value obtained with 30 wt.% of ceramic content, followed by a gradual decrease, and eventually no deposition at 100% Al. The experiment demonstrated the positive impact of the presence of ceramic particles in improving the DE of Al coating.
Nowadays, energy, information and materials have become the symbol of the progress of human civilization, and materials are the important material basis for human survival and development. Following the metal, ceramics, polymer materials, cermet materials are with its outstanding performance, a wide range of varieties and a wide range of uses into all walks of life.
1. What is Cermet?
Cermet is a structural material composed of ceramic hard phase bonded to metal or alloy. Cermet not only maintains high strength, high hardness, wear resistance, high-temperature resistance, oxidation resistance and chemical stability of ceramics, but also has good mental toughness and plasticity.
The characteristics of cermet mainly include the following aspects:
(1) The wettability of metal to ceramic phase is good.
The wettability between metal and ceramic particles is one of the main conditions to evaluate the microstructure and properties of cermet. The stronger the wetting ability is, the more likely the metal forms a continuous phase, and the better the cermet is.
(2) There is no severe chemical reaction between the metal phase and the ceramics.
If the interfacial reaction is intense and the compound is formed in the preparation of cermet, it is impossible to improve the resistance of ceramics to mechanical shock and thermal shock by using metal phase.
(3) The expansion coefficient between the metal phase and the ceramic phase will not be too large.
When the expansion coefficients of the cermet and metal phases differ greatly, the internal stress will be increased and the thermal stability of cermet will be reduced.
2. Preparation methods of cermets
The preparation methods of cermet include hot pressing, powder sintering and impregnation.
3. Classification of cermets
3.1 Oxide based cermets
Oxide-based cermets are composed of alumina, zirconia, magnesium oxide, beryllium oxide and tungsten, chromium or cobalt. They are characterized by high-temperature resistance, chemical corrosion resistance, good thermal conductivity and high mechanical strength.
The wettability between Cr and Al2O3 is not good, but a dense layer of Cr2O3 is easily formed on the surface of metal chromium powder, so the interfacial energy between them can be reduced and the wettability can be improved by forming Al2O3-Cr2O3 solid solution. In order to make the metal chromium oxidized partially, some measures are often adopted, such as introducing trace water vapor or oxygen into the sintering atmosphere, replacing alumina with a part of Al (OH) 3 in the batching, and replacing metal chromium with a part of chromium oxide in the batching. Al2O3-Cr cermets are made from 99.5% purity of a-Al2O3 and 99% purity of electrolytic Cr powder. Al2O3 and Cr powder are dried or wet ground together to the necessary size composition, which can be formed by any molding method.
3.2 carbide based cermets
Carbide based cermets. Titanium carbide, silicon carbide, tungsten carbide and other metals as the matrix, and metal cobalt, nickel, chromium, tungsten, molybdenum composite, with high hardness, high wear resistance, high temperature and other characteristics. Here is a brief introduction to titanium carbide (TiC) cermets.
TiC has the high melting point, high hardness, high elastic modulus, good thermal shock resistance and chemical stability, and its high-temperature oxidation resistance is only lower than that of SiC. Titanium carbide is an important raw material of cemented carbide, so it is widely used as a hard phase in structural materials to make titanium carbide-based cermets such as wear-resistant materials, cutting tool materials, mechanical parts, etc. It is a heterogeneous composite material composed of metal or alloy with titanium carbide ceramic phase, which keeps the ceramic high. The strength, hardness, wear resistance, high-temperature resistance, oxidation resistance and chemical stability are also good. Because of these excellent physical and chemical properties, titanium carbide based cermets have attracted much attention.
3.3 titanium nitride-based cermets
In , Ford Motor Company discovered that adding molybdenum alloy to TiC-Ni based cermets could improve the wettability of Ni to TiC and greatly enhance the strength of the alloy. In , Kieffer et al. found that the addition of TiN into TiC-Mo-Ni cermets could not only significantly refine the hard phase grains, improve the mechanical properties of Cermets at room and high temperature, but also greatly improve the high-temperature corrosion resistance and oxidation resistance of cermets. Therefore, TiC(N)) cermets based on titanium carbide nitride were very popular at home and abroad. Attention has been made and systematic studies have been carried out. Since the s, Ti (C, N) based cermets have developed rapidly. Cemented carbide manufacturers all over the world have introduced a series of Ti (C, N) based cermets tools. Over the past 30 years, with the development of powder metallurgy technology, composition evolution tends to be stable, sintering technology is constantly updated, powder size is constantly refined, Ti (C, N) based cermet has developed to a relatively mature stage.
3.4 boride cermet
Boride ceramics are interstitial compounds. Many complex covalent bonds can be formed between boron and boron. At the same time, boron can form ion bonds with many metal atoms. This characteristic determines that boride has high melting point, high hardness, high wear resistance and high corrosion resistance, so it is widely used in cemented carbide materials and wear resistant materials. In boride ceramics, binary borides such as TiB2, ZrB2 and CrB2 are considered as the most promising boride ceramics because of their excellent properties. However, due to the strong chemical reaction between binary boride ceramics such as TiB2 and metal matrix, the sintering performance will deteriorate.
The research on the practical application of three element boride cermet in the industrial field remains to be further studied. The existing problems include:
(1) Because ternary boride cermet mainly uses molybdenum powder, ferroboron alloy powder, nickel powder and chromium powder as main raw materials, the production cost is high.
(2) The reliability and reproducibility of the three element boride cermet are poor.
4. Application of Cermet
(1) Cutting area
Cermet tools have high hardness, red hardness and wear resistance, and excellent cutting performance in high-speed cutting and dry cutting. Under the same cutting conditions, the wear resistance of cermet tools is much higher than that of ordinary cemented carbide.
(2) Aerospace Industry
TiC-Ni cermets have been used as high-temperature materials for jet engine blades since the s. However, TiC particles agglomerate and grow up during sintering because nickel can not completely wet TiC, which results in poor toughness of the materials and fails to be used as heat-resistant materials. TiC itself has high hardness, high melting point, low specific gravity and good thermal stability, while copper has excellent electrical conductivity, thermal conductivity and good plasticity. TiC/Cu composites composed of TiC and metallic copper synthesize the excellent properties of both and have the application as conductive, thermally conductive, wear-resistant materials and materials for rocket throat lining.
(3) Other applications
Cermet composite coating can change the appearance, structure and chemical composition of the outer surface of the metal matrix, and give the matrix new properties. Cermet composite coating is a kind of excellent composite material with the advantages of strength and toughness of metal and high-temperature resistance of ceramics. It has been successfully applied to aerospace, aviation, national defense, chemical industry, machinery, power, and electronics industries.
The ceramic lined composite pipe has better performance than ceramic lined pipe. Self-propagating high-temperature synthesis centrifugal casting of liner ceramics can be used as corrosion-resistant pipelines for transportation of petroleum or chemical products and semi-products, as anti-wear pipelines for mines, as slurry transportation pipelines in ore dressing plants, and as water pipelines with muddy sand.
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The polymer-derived ceramic (PDC) composites are an interesting category of materials with unique properties compared to conventional ceramic materials. The transformation of polymer to ceramic helps in achieving significant breakthroughs in ceramic technology. The composites find applications in wide areas, including aerospace, nuclear, and defense industries, where the oxidation resistance and high-temperature properties of PDC are beneficial [ 74 76 ]. The capability to fabricate complex shapes with the PDC enhances the application areas for this processing route. The advancements in this area have been in obtaining PDC as coatings, fibers, reinforcements, or ceramics which are stable at ultra-high temperatures (up to °C). However, the synthesis of cermets through the PDC route is viable and can be further investigated in the future.
Liquid-phase sintering is effective when there is good wettability and solubility of the hard phase in the liquid. The absence of such a mechanism in most cermet systems requires other processes, such as pressure-assisted sintering (hot uniaxial or isostatic pressing) to obtain pore-less materials in non-wetting systems, such as oxide cermets. These techniques are carried out at relatively low temperatures to eliminate the formation of undesirable phases. Spark plasma sintering is a recent development for sintering at relatively low temperatures within a short period. The infiltration of molten metal into the ceramic is another development in the synthesis of cermet systems. This method overcomes the limitation of the excessive shrinkage associated with liquid-phase sintering and produces accurate dimensions in complex shapes. The infiltration system is most effective with a wetting system where it penetrates the matrix by capillary action. A pressure-assisted infiltration system is employed for metal matrix composite fabrication, and can be utilized in the fabrication of cermets, but the high-volume content of ceramic in the matrix makes this process difficult. The infiltration technique can also be applied to fabricate functionally graded cermets.
In most cases, the cermets possess a typical core-shell morphology. The core consists of the undissolved particles, whereas the shell is produced by sintering through reaction and precipitation. The core and shell possess identical crystal orientations. The ceramic particles join each other and form a continuous network during the sintering process to minimize surface energy. A similar interconnected network formation also occurs with metallic binders. The metallic binders in this constrained state exhibit features different from those of the free metal [ 73 ]. The distribution of the network of metallic binders and the formation and characteristics of ceramic particles within the cermet system greatly influence the final properties of the system.
In many cases, an additional shaping operation is required to obtain the final shape. The cold compacted product is then sintered with fully dense cermets obtained using a high-temperature PM process. This is generally performed in ovens under an inert atmosphere, vacuum, or in hydrogen. Depending on the ceramic-metal system, liquid or solid-phase sintering is performed. The liquid-phase sintering enhances densification by rearranging particles, grain coarsening, and solution precipitation [ 72 ]. Following the sintering process, hot isostatic pressing (HIP) may be carried out to eliminate residual porosities. The solid-state sintering process achieves bonding and densification by applying heat below the melting point of the materials. Solid-state sintering processes, such as powder-rolling and warm extrusion, are utilized to produce wires or slabs. Figure 2 depicts the steps involved in the PM processing of cermets. The sintering process leads to building up of the microstructure, morphology, and phases of the cermet.
The most common methods of cermet fabrication involve PM techniques. The main steps involved in PM processes are the mixing of powder, milling of the powder mix to obtain proper intermixing and the desired particle size, compaction, and sintering. The powders of ceramic and metal particles are thoroughly mixed and further milled in ball mills or attritors. To prevent the agglomeration of the powder and oxidation, a wetting organic lubricant, such as polyglycol or paraffin wax, is added along with the mixture. The milling process facilitates the uniform distribution of ceramic grain sizes and embeds the ceramic particles with metal, which aids in the sintering process. The formed slurry is dried to expel the solvent, and, eventually, the powder forms as spherical granules with dimensions in the range of 0.1 mm&#;0.5 mm [ 5 ]. The milling process is followed by the powder compaction step. The part is formed into the required shape during this step. Cold pressing is the most commonly used method for relatively simpler shapes. Complex shapes can be achieved by PM injection molding, cold isostatic pressing, or extrusion slip casting.
Single and multiphase materials with oxide and non-oxide ceramics, intermetallics, and metals can be synthesized using RHP. The powder preformed in the porous starting condition is transformed into desirable phases involving reactions such as reduction reactions, displacement reactions, and elemental precursors reactions [ 81 ]. Cermets that include Al/Ni and Al/Nb are synthesized with this method. LaSalvia et al. [ 82 ] have successfully synthesized TiC-30% Ni and studied the effect of varying the Mo addition on its microstructural and mechanical properties.
The reaction synthesis process, owing to its ability to produce CMCs with desired microstructural features and tailored properties, has recently attracted much attention as a fabrication technique for CMCs. The various reaction synthesis processes, including directed metal oxidation (DMO), reactive metal penetration (RMP), reaction bonding (RB), reactive hot pressing (RHP) and reactive forging (RF) can be used to fabricate cermets with a typical three-dimensionally interconnected ceramic reaction product with some metallic content. An alumina ceramic matrix with residual Al metal can be achieved with the DMO approach. The DMO process has been implemented successfully for several cermet systems, including carbides, nitrides, borides, and oxides of Al, Ti, Zr, Hf, and Si [ 77 80 ].
Wayne et al. [ 97 ] analyzed the microstructure, mechanical, and wear properties of thermally sprayed and sintered WC-Co cermet systems with two different types of binder content. The influence of cobalt content was not significant in the wear resistance to diamond abrasion of the thermally sprayed cermet. However, it showed a greater effect in controlling the wear resistance due to particle erosion. The abrasion wear and erosion wear resistance of the studied cermet system mainly depended on the porosity, mean free path, and carbide grain size of the binder. It was evident from the observations that, for coatings, the porous structure resulted in poor intersplat bonds, which further reduced the hardness and fracture toughness. These observations are in contrast to those of other researchers because of the difference in the tribological conditions used.
The spraying of WC-based cermet coatings is one of the most significant applications of the HVOF method because of the reduced carbide transformation due to the low temperature received by the particles. High particle velocities are attained using a converging-diverging de Laval nozzle design and high gas pressures. Solid particle erosion (SPE) plays a critical role in the degradation of materials. Cermet systems based on WC and chromium carbide (CrC) coated by HVOF have shown impressive erosion behavior [ 93 94 ]. Kumar et al. [ 95 ] experimented on WC and CrC-based HVOF thermally sprayed cermet coatings to study their erosion behavior and investigated their effectiveness in the application of pulverized coal burner nozzles (PCBN) to mitigate SPE. It was observed that the coatings failed to sustain the erosion attacks and exposed the substrate. One possible reason for this was the larger particle velocity and the flux incorporated to mimic the PCBN conditions. It is still possible for the coatings to sustain particles moving in low velocities but they fail to do so in applications involving high velocity together with high particle flux. Wood et al. [ 96 ] investigated the tribology of thermally sprayed WC-Co coatings. They observed a more than 50% improvement in erosion resistance for HVOF coatings after parameter optimization compared to coating achieved with a conventional D-gun with similar nominal composition. The abrasive resistance of these coatings was comparable to sintered cermets having the same composition. The coating was found to have relatively high dry sliding and wet sliding wear resistance with a COF ranging from 0.2 to 0.5. The friction and wear appeared to be influenced by the oxide presence on the binder.
HVOF is a TS coating technique that can produce coatings with a very dense and compact structure and good adhesion to the substrate. Recent advancements in HVOF spraying have made it possible to produce coatings with lower porosities and decarburization [ 91 ]. The HVOF technique produces an efficient deposit of composite coatings with higher density, good bond strength, and comparatively lower decarburization. This is because of the high particle velocities and relatively lower temperature during deposition. In the HVOF process, the fuel and oxygen are intermixed and burnt in the combustion chamber at higher flow rates of up to L/min, and with a pressure range of up to 12 bars. This produces a high-speed gas jet. Powder particles with a size in the range of 5&#;70 µm are injected into the high-speed gas jet and accelerated towards the substrate. The heated powders are deposited on the substrate at 600&#;650 m/s. Upon impact, they form lenticular splats and adhere strongly to each other and the substrate. Raster movement of the HVOF gun in several passes achieves the required coating thickness [ 92 ].
The TS coating is performed by melting materials, which are in wire stock or particulate form, and accelerating the partially melted or fully melted particle droplets towards a substrate. As the droplets strike the substrate, they expand out radially to form a &#;splat&#;. With continued deposition of these splats, they eventually interact and combine to form a continuous coating. High particle velocities or high temperature during the impact can improve the bonding between these splats and the removal of pores. Various processes have varying thermal and kinetic energy contents [ 88 ]. TS can produce metallic, ceramic, carbide, and cermet coatings with any phase composition on a properly prepared base. In general, TS coatings possess good adhesion strength with various substrates. They have good wear resistance and low corrosion rates. Typically, these are applied to weaker underlying materials to ensure improved wear properties, thereby increasing the life of the components [ 89 ].
Thermal spray (TS) processes are growing rapidly and represent an important surface modification technology [ 83 ]. They have become an important modern industrial tool capable of producing customized surface properties for a range of industrial applications, which include thermal barrier coatings for turbine blades, erosion-resistant coatings for boiler tubes, and so on [ 84 ]. Like most coating techniques, TS coating helps in combining the advantages of the core with increased hardness, resistance to abrasive wear, and heat resistance [ 85 ]. TS is one of the most frequently used protective coating technologies. The TS technique is carried out by partially or completely melting the spraying material for milliseconds, then propelling it onto the surface to be coated with highly accelerated velocities [ 86 ]. The microstructure and properties of the thermally sprayed components depend on the powder characteristics and processing parameters [ 87 ].
Cold spraying (CS) or cold gas dynamic spraying is a process where solid powders are deposited on a substrate using a de Laval nozzle. To explore specific properties, CS can effectively deposit various materials, including metals, ceramics, polymers, and composites. As the particle&#;s impact velocity exceeds a threshold limit, it undergoes plastic deformation and adheres to the substrate [ 98 ]. Because of the low gas temperatures arising due to the rapid gas expansion in the nozzle, the feedstock powders stay solid throughout the complete travel of the nozzle [ 71 ]. The CS technique can form very dense coatings with very low oxygen content without grain growth, residual tensile stresses, or phase changes. The deposition of some materials can even produce grain refinement at the nanoscale. These properties make CS suitable for the deposition of a set of advanced materials. This is an efficient and novel technique to produce surface coatings with several advantages over TS. It uses the particle&#;s kinetic energy instead of thermal energy for deposition. Through this, oxidation, undesired chemical reactions, and tensile residual stress can be eliminated [ 99 ]. The low temperatures in CS lead to the occurrence of unique characteristics. The CS technique can retain the microstructure and properties of the feedstock powders and avoid oxide formation and undesirable structural changes. This helps in improving the durability of the coating. CS coatings are used to deposit not only metals, as was its initial purpose, but also polymers, ceramics, and advanced composites. As the particles contact the substrate, they experience severe plastic deformation due to the kinetic energy released. They are adhered to the substrate by mechanical anchoring. If there is sufficient plastic deformation, metallurgical bonding facilitates adherence [ 98 101 ]. In the conventional case of cold spraying of metals on metallic substrates, the adhesion between the substrate and the metal occurs due to the combined effect of metallurgical bonding and interlocking via the adiabatic shear instability (ASI) mechanism [ 98 ].
There are two types of CS systems: high-pressure cold spray (HPCS) and low-pressure cold spray (LPCS) systems. In the HPCS system, particles are injected ahead of the nozzle throat from a high-pressure gas supply. In contrast, in the LPCS system, the powder particles are injected into the diverging section of the nozzle by a low-pressure gas supply. In high-pressure cold spraying ( Figure 3 ), a preheated high-pressure gas (up to psi), either nitrogen or helium, is forced through a converging-diverging de Laval nozzle. The nozzle converts enthalpy into kinetic energy by expansion and accelerates the gas flow to a supersonic region ( m/s), causing a temperature reduction. The powder is supplied axially into the gas stream before reaching the nozzle throat. The gas carries the powder particles with sufficient acceleration and impacts the substrate. The particle&#;s kinetic energy is sufficient to induce a mechanical or metallurgical bond between particles and the substrate [ 99 102 ].
In low-pressure cold spraying ( Figure 4 ), air or nitrogen is used as the carrier gas with relatively low-pressure values (80&#;140 psi). The preheated gas is forced through a de Laval nozzle and accelerated to 600 m/s. Then, the powder is supplied into the nozzle&#;s diverging section (downstream) and accelerated towards the substrate [ 103 ].
104,105,The cold spraying of cermets is performed by mixing the reinforcement particles with the ductile metallic feedstock powders. Materials such as ceramics, oxides, and so on, in their pure form, cannot produce a coating on any surface without causing surface erosion. However, numerous studies have reported the possibility of depositing already prepared cermet mixtures on different substrates [ 68 106 ]. By this method, metal coatings with ceramic inclusions can be produced. Only a limited amount of the reinforcement particles are retained in the final coating. The process does not induce severe plastic deformation to the hard reinforcement particles as is the case with ductile materials, but rather leads to more plastic deformation in ductile materials. These particles embed themselves in the coating, producing cermets [ 68 70 ].
107,Several ceramic-metal coating combinations have been successfully obtained using the CS technique [ 70 108 ]. The intended properties and applications of these coatings are almost the same as in the case of TS techniques. These mainly include wear resistance, high hardness, and high-temperature hardness. The addition of ceramic particles is the main reason for enhancing these properties. It also influences the deposition process behavior. There are three main theories that have been proposed to explain the influence of ceramic particles on the coating and deposition properties. One proposed mechanism involves the peening or impingement effect of ceramic particles [ 109 ]. It is proposed that, as the spraying happens, the hard ceramic particles behave like shot balls, which peen the softer particles in front of them, causing more deformation. This, in turn, increases the DE of softer metallic particles. This mechanism has also been described in several other studies [ 107 110 ].
Irissou et al. [ 70 111 ] proposed another mechanism for the improved DE of the CS technique. They observed that as the ceramic content in the feedstock powder mixture increases, the roughness of the interface between the coating and the interface increases. The increase in roughness is attributed to the grit blasting action of the impacting ceramic particles. In addition, they suggest that the asperities formed during this process help to bond more particles, thereby improving the DE due to mechanical anchoring. The third mechanism suggested is related to the oxide cleaning action of ceramic particles on metallic particles or substrates. The ceramic particles, upon impacting the metallic particles or substrates, remove the oxide films, thereby exposing the fresh surface of the material for the further impact of metallic particles. This creates favorable bonding sites. The same effect can be achieved by the pure deformation of the brittle oxide layer, shredding from the surface, and improving the DE [ 112 113 ]. The optimization of parameters serves a crucial role in producing quality CS materials. Couto et al. [ 114 ] experimented with studying the wear and corrosion properties of WC-based cermets with two distinct binder proportions (WC-7Co and WC-12Co) coated on an Al -T6 aluminum substrate using the CS technique. They optimized the critical parameters influencing the properties of the deposited coating, including the temperature, spraying angle, spraying distance, gas pressure, and gas medium. They were successful in producing a dense and well-bonded coating on the substrate. Significant features of the processing route included that the coating obtained after spraying had no microstructural changes, decarburization, or any unwanted phase formation, indicating that the bulk properties of the feedstock powder were preserved. It was observed that higher temperature resulted in denser and thicker coatings on both occasions for the WC-Co cermet.
2O3 and SiC). This also led to an increase in the DE of the metal component of the mixture compared to the deposition of pure metal alone. The coarser ceramic powders negatively influenced the process by creating a strong erosion effect, which considerably reduced the deposition efficiency of the metal over the substrate. The deposition behavior and DE for several Al-Al2O3 cermet systems were investigated by Fernandez et al. [2O3 feedstock powders to analyze the deposition behavior and influence of the ceramic content on deposition efficiency. An increase in DE was observed, with a peak value obtained with 30 wt.% of ceramic content, followed by a gradual decrease, and eventually no deposition at 100% Al2O3. The experiment demonstrated the positive impact of the presence of ceramic particles in improving the DE of Al coating.Dosta et al. [ 115 ] ] conducted a similar study of the wear and corrosion resistance of WC-25Co cermet cold sprayed on carbon steel and Al-T6. They measured the bonding strength of the coating by adhesion testing based on ASTM C633-08. Sliding and abrasive resistance were also measured. The main objective of this study was to optimize the spraying conditions of the cermet system and the CS system to obtain good quality coatings. Dense and thick coatings on both substrates were obtained using the CS technique. The coating obtained had no decarburization, microstructural change, or unwanted phases. The CS technique is promising and can produce better coatings than the TS coating technique. A major concern associated with ceramic particles in the CS technique is the erosion that might occur to the metallic particles or substrate. The influence of particle size on the properties of cermet coatings was studied experimentally by Sova et al. [ 68 ]. They experimented with a specially developed nozzle with a separate metal powder and ceramic injection into the gas stream. They observed a strong activation effect and better coating for spraying soft metals (Al, Cu) while incorporating them with fine ceramic powders (Aland SiC). This also led to an increase in the DE of the metal component of the mixture compared to the deposition of pure metal alone. The coarser ceramic powders negatively influenced the process by creating a strong erosion effect, which considerably reduced the deposition efficiency of the metal over the substrate. The deposition behavior and DE for several Al-Alcermet systems were investigated by Fernandez et al. [ 71 ]. They used different Al-Alfeedstock powders to analyze the deposition behavior and influence of the ceramic content on deposition efficiency. An increase in DE was observed, with a peak value obtained with 30 wt.% of ceramic content, followed by a gradual decrease, and eventually no deposition at 100% Al. The experiment demonstrated the positive impact of the presence of ceramic particles in improving the DE of Al coating.
Nowadays, energy, information and materials have become the symbol of the progress of human civilization, and materials are the important material basis for human survival and development. Following the metal, ceramics, polymer materials, cermet materials are with its outstanding performance, a wide range of varieties and a wide range of uses into all walks of life.
1. What is Cermet?
Cermet is a structural material composed of ceramic hard phase bonded to metal or alloy. Cermet not only maintains high strength, high hardness, wear resistance, high-temperature resistance, oxidation resistance and chemical stability of ceramics, but also has good mental toughness and plasticity.
The characteristics of cermet mainly include the following aspects:
(1) The wettability of metal to ceramic phase is good.
The wettability between metal and ceramic particles is one of the main conditions to evaluate the microstructure and properties of cermet. The stronger the wetting ability is, the more likely the metal forms a continuous phase, and the better the cermet is.
(2) There is no severe chemical reaction between the metal phase and the ceramics.
If the interfacial reaction is intense and the compound is formed in the preparation of cermet, it is impossible to improve the resistance of ceramics to mechanical shock and thermal shock by using metal phase.
(3) The expansion coefficient between the metal phase and the ceramic phase will not be too large.
When the expansion coefficients of the cermet and metal phases differ greatly, the internal stress will be increased and the thermal stability of cermet will be reduced.
2. Preparation methods of cermets
The preparation methods of cermet include hot pressing, powder sintering and impregnation.
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3. Classification of cermets
3.1 Oxide based cermets
Oxide-based cermets are composed of alumina, zirconia, magnesium oxide, beryllium oxide and tungsten, chromium or cobalt. They are characterized by high-temperature resistance, chemical corrosion resistance, good thermal conductivity and high mechanical strength.
The wettability between Cr and Al2O3 is not good, but a dense layer of Cr2O3 is easily formed on the surface of metal chromium powder, so the interfacial energy between them can be reduced and the wettability can be improved by forming Al2O3-Cr2O3 solid solution. In order to make the metal chromium oxidized partially, some measures are often adopted, such as introducing trace water vapor or oxygen into the sintering atmosphere, replacing alumina with a part of Al (OH) 3 in the batching, and replacing metal chromium with a part of chromium oxide in the batching. Al2O3-Cr cermets are made from 99.5% purity of a-Al2O3 and 99% purity of electrolytic Cr powder. Al2O3 and Cr powder are dried or wet ground together to the necessary size composition, which can be formed by any molding method.
3.2 carbide based cermets
Carbide based cermets. Titanium carbide, silicon carbide, tungsten carbide and other metals as the matrix, and metal cobalt, nickel, chromium, tungsten, molybdenum composite, with high hardness, high wear resistance, high temperature and other characteristics. Here is a brief introduction to titanium carbide (TiC) cermets.
TiC has the high melting point, high hardness, high elastic modulus, good thermal shock resistance and chemical stability, and its high-temperature oxidation resistance is only lower than that of SiC. Titanium carbide is an important raw material of cemented carbide, so it is widely used as a hard phase in structural materials to make titanium carbide-based cermets such as wear-resistant materials, cutting tool materials, mechanical parts, etc. It is a heterogeneous composite material composed of metal or alloy with titanium carbide ceramic phase, which keeps the ceramic high. The strength, hardness, wear resistance, high-temperature resistance, oxidation resistance and chemical stability are also good. Because of these excellent physical and chemical properties, titanium carbide based cermets have attracted much attention.
3.3 titanium nitride-based cermets
In , Ford Motor Company discovered that adding molybdenum alloy to TiC-Ni based cermets could improve the wettability of Ni to TiC and greatly enhance the strength of the alloy. In , Kieffer et al. found that the addition of TiN into TiC-Mo-Ni cermets could not only significantly refine the hard phase grains, improve the mechanical properties of Cermets at room and high temperature, but also greatly improve the high-temperature corrosion resistance and oxidation resistance of cermets. Therefore, TiC(N)) cermets based on titanium carbide nitride were very popular at home and abroad. Attention has been made and systematic studies have been carried out. Since the s, Ti (C, N) based cermets have developed rapidly. Cemented carbide manufacturers all over the world have introduced a series of Ti (C, N) based cermets tools. Over the past 30 years, with the development of powder metallurgy technology, composition evolution tends to be stable, sintering technology is constantly updated, powder size is constantly refined, Ti (C, N) based cermet has developed to a relatively mature stage.
3.4 boride cermet
Boride ceramics are interstitial compounds. Many complex covalent bonds can be formed between boron and boron. At the same time, boron can form ion bonds with many metal atoms. This characteristic determines that boride has high melting point, high hardness, high wear resistance and high corrosion resistance, so it is widely used in cemented carbide materials and wear resistant materials. In boride ceramics, binary borides such as TiB2, ZrB2 and CrB2 are considered as the most promising boride ceramics because of their excellent properties. However, due to the strong chemical reaction between binary boride ceramics such as TiB2 and metal matrix, the sintering performance will deteriorate.
The research on the practical application of three element boride cermet in the industrial field remains to be further studied. The existing problems include:
(1) Because ternary boride cermet mainly uses molybdenum powder, ferroboron alloy powder, nickel powder and chromium powder as main raw materials, the production cost is high.
(2) The reliability and reproducibility of the three element boride cermet are poor.
4. Application of Cermet
(1) Cutting area
Cermet tools have high hardness, red hardness and wear resistance, and excellent cutting performance in high-speed cutting and dry cutting. Under the same cutting conditions, the wear resistance of cermet tools is much higher than that of ordinary cemented carbide.
(2) Aerospace Industry
TiC-Ni cermets have been used as high-temperature materials for jet engine blades since the s. However, TiC particles agglomerate and grow up during sintering because nickel can not completely wet TiC, which results in poor toughness of the materials and fails to be used as heat-resistant materials. TiC itself has high hardness, high melting point, low specific gravity and good thermal stability, while copper has excellent electrical conductivity, thermal conductivity and good plasticity. TiC/Cu composites composed of TiC and metallic copper synthesize the excellent properties of both and have the application as conductive, thermally conductive, wear-resistant materials and materials for rocket throat lining.
(3) Other applications
Cermet composite coating can change the appearance, structure and chemical composition of the outer surface of the metal matrix, and give the matrix new properties. Cermet composite coating is a kind of excellent composite material with the advantages of strength and toughness of metal and high-temperature resistance of ceramics. It has been successfully applied to aerospace, aviation, national defense, chemical industry, machinery, power, and electronics industries.
The ceramic lined composite pipe has better performance than ceramic lined pipe. Self-propagating high-temperature synthesis centrifugal casting of liner ceramics can be used as corrosion-resistant pipelines for transportation of petroleum or chemical products and semi-products, as anti-wear pipelines for mines, as slurry transportation pipelines in ore dressing plants, and as water pipelines with muddy sand.
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