Effect of Ceramic Particles on Properties of Cold-Sprayed Ni-20Cr+Al2O3 Coatings

Cold spraying is a thermal spray process enabling the production of metallic and metal-ceramic coatings with low porosity and low oxygen content, capable of, e.g., resisting corrosion. The aim of this study was to characterize the microstructural and mechanical properties of cold-sprayed Ni-20Cr+Al₂O₃ coatings and to clarify the effect of the hard particles on different coating properties. Accordingly, the research focused on the microstructure, denseness (impermeability), adhesion strength, and hardness of the coatings. Scanning electron microscopy (SEM) analysis and corrosion tests were run to gain information on the through-porosity. Ceramic addition in cold-sprayed Ni-20Cr+Al₂O₃ coatings improved their quality by lowering their porosity. Moreover, hardness was slightly higher than those of cold-sprayed Ni-20Cr coating, indicating a hardening effect by the ceramic particles. The addition of Al₂O₃ also made it possible to use high gas temperatures without nozzle clogging, which affects coating properties, such as coating thickness, denseness, and hardness.     

Effect of applied current on the electrodeposited Ni-Al2O3 composite coatings

Nano-sized Al₂O₃ ceramic particles (50 nm) were co-deposited with nickel using electrodeposition technique to develop composite coatings. The coatings were produced in an aqueous nickel bath at different current densities and the research investigated the effect of applied current on microstructure and thickness of the coatings. The variation in some mechanical properties such as hardness, wear resistance, and the adhesive strength of the composite coatings is influenced by the applied current and this was also studied. The morphology of the coatings was characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The hardness, wear resistance, and bond strength of the coatings were evaluated by Vickers micro-hardness test, pin-on-disc test, and tensile test, respectively. Results showed that the Al₂O₃ particles were uniformly distributed in the coatings, and the coatings deposited at a current density of 0.01 A/cm² was most favorable in achieving a maximum current efficiency which causes the co-deposition of a maximum amount of Al₂O₃ particles (4.3 wt.%) in the coatings. The increase in Al₂O₃ particles in the coatings increased the mechanical properties of the Ni-Al₂O₃ composite coatings by grain refining and dispersion strengthening mechanisms.     

Consolidation of Al2O3/Al Nanocomposite Powder by Cold Spray

While the improvement in mechanical properties of nanocomposites makes them attractive materials for structural applications, their processing still presents significant challenges. In this article, cold spray was used to consolidate milled Al and Al₂O₃/Al nanocomposite powders as well as the initial unmilled and unreinforced Al powder. The microstructure and nanohardness of the feedstock powders as well as those of the resulting coatings were compared. The results show that the large increase in hardness of the Al powder after mechanical milling is preserved after cold spraying. Good quality coating with low porosity is obtained from milled Al. However, the addition of Al₂O₃ to the Al powder during milling decreases the powder and coating nanohardness. This lower hardness is attributed to non-optimized milling parameters leading to cracked particles with insufficient Al₂O₃ embedding in Al. The coating produced from the milled Al₂O₃/Al mixture also showed lower particle cohesion and higher amount of porosity.     

The influence of ceramic particles on bond strength of cold spray composite coatings on AZ91 alloy substrate

Al-Al₂O₃ composite coatings were produced on AZ91D magnesium alloy substrates using kinetic metallization (KM), which is a special type of cold spray using a convergent barrel nozzle to attain sonic velocity. The effect of the volume fraction of Al₂O₃ particles and KM spray temperatures on the microstructure, hardness of the composite coatings, the deposition efficiency, and the bond strength between the coating and substrate was studied. Results show that addition of Al₂O₃ particles not only significantly improves the density of the coating, but also enhances the deposition efficiency to an optimum value. The bond strength of the composite coatings with the substrate was found to be much stronger than the coating itself, measured using a specially designed lug shear method. Furthermore, based on bond strength data and SEM analysis, higher Al₂O₃ content resulted in a failure mode transition from adhesive failure to cohesive failure. This is considered a result of a competition between the strengthening of the ceramic reinforcing particles at the coating/substrate interface, and the weakening of coating cohesive strength due to an increase in the proportion of weaker Al-Al₂O₃ bonds compared with stronger Al-Al bonds. Characterisation of the composite coating in terms of hardness, porosity and microstructure was also conducted.     

Effect of Milling Time on Electrical Breakdown Behavior of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Cu Composite

In order to clarify the relationship between the microstructure and the arc erosion behavior of metal-matrix composite, Al₂O₃/Cu composites with different distributions of Al₂O₃ particles were prepared by high energy ball milling and powder metallurgy. The effect of milling time on microstructure, properties, and arc erosion behavior of Al₂O₃/Cu composite was investigated. The results show that the distribution of Al₂O₃ particles improves significantly with increase of milling time, but Al₂O₃ particles will be aggregated if milling time is too long. The optimal milling time is 24 h in the range of experiments. A uniform distribution of Al₂O₃ particles in copper matrix can improve the hardness, electrical conductivity, average breakdown strength, chopping level, and arc life. With improvement in the distribution of Al₂O₃ particles, the erosion area becomes larger, and the erosion pits become shallower and are dispersed more uniformly.     

Effect of Feedstock Size and its Distribution on the Properties of Detonation Sprayed Coatings

The detonation spraying is one of the most promising thermal spray variants for depositing wear and corrosion resistant coatings. The ceramic (Al₂O₃), metallic (Ni-20 wt%Cr) , and cermets (WC-12 wt%Co) powders that are commercially available were separated into coarser and finer size ranges with relatively narrow size distribution by employing centrifugal air classifier. The coatings were deposited using detonation spray technique. The effect of particle size and its distribution on the coating properties were examined. The surface roughness and porosity increased with increasing powder particle size for all the coatings consistently. The feedstock size was also found to influence the phase composition of Al₂O₃ and WC-Co coatings; however does not influence the phase composition of Ni-Cr coatings. The associated phase change and %porosity of the coatings imparted considerable variation in the coating hardness, fracture toughness, and wear properties. The fine and narrow size range WC-Co coating exhibited superior wear resistance. The coarse and narrow size distribution Al₂O₃ coating exhibited better performance under abrasion and sliding wear modes however under erosion wear mode the as-received Al₂O₃ coating exhibited better performance. In the case of metallic (Ni-Cr) coatings, the coatings deposited using coarser powder exhibited marginally lower-wear rate under abrasion and sliding wear modes. However, under erosion wear mode, the coating deposited using finer particle size exhibited considerably lower-wear rate.     

Effect of micro-blasting on the tribological properties of TiN/MT-TiCN/Al2O3/TiCNO coatings deposited by CVD

The effect of micro-blasting on the tribological properties of TiN/MT-TiCN/Al₂O₃/TiCNO coatings was studied. The multilayer coatings were deposited on cemented carbides by chemical vapor deposition. The microstructure, mechanical and tribological properties were investigated using X-ray diffraction, scanning electron microscopy (SEM), nano-mechanical testing system, scratch tester and reciprocating tribometer. The results show that micro-blasting significantly reduces the surface roughness and converts the residual tensile stress of Ti(C,N,O) top-layer and Al₂O₃ layer into compressive stress. Affected by the residual compressive stress, the hardness and adhesion strength are increased. More importantly, the friction coefficient is decreased attributed to the decreased surface roughness and improved hardness. Also, the wear resistance of micro-blasted TiN/MT-TiCN/Al₂O₃/TiCNO is superior due to higher hardness of Ti(C,N,O) top-layer, Al₂O₃ layer and adhesion strength of coatings. Especially for the total sliding time of 2 h, the wear volume and wear rate of micro-blasted coatings are 69.4% of as-deposited coatings, because micro-blasting helps to increase the adhesion strength and micro-cracking resistance, which play important roles in the improvement of wear resistance. Micro-blasting has a positive effect on the friction and wear properties of TiN/MT-TiCN/Al₂O₃/TiCNO multilayer coatings since the adverse impact of top-layer thinning is offset.     

Comparison of 10 μm and 20 nm Al-Al2O3 Metal Matrix Composite Coatings Fabricated by Low-Pressure Cold Gas Dynamic Spraying

Cold-gas dynamic spraying (“cold spraying”) was used to deposit aluminum-alumina (Al-Al₂O₃) metal-matrix composite (MMC) coatings onto 6061 Al alloy. The powders consisted of ?45 μm commercially pure Al that was admixed with either 10 μm or agglomerated 20 nm Al₂O₃ in weight fractions of 25, 50, 75, 90, and 95 wt.%. Scanning electron microscopy (SEM), Vickers microhardness testing, and image analysis were conducted to determine the microstructure, properties, and the volume fractions of reinforcing particles in the coatings, which was then converted to weight fractions. As the weight fraction of the Al₂O₃ in the coatings increased, the hardness values of the MMC coatings increased. A maximum hardness of 96 ± 10 HV_0.2 was observed for the MMC coating that contained the agglomerated 20 nm Al₂O₃ particles, while a maximum hardness of 85 ± 24 HV_0.2 was observed for the coatings with the 10 μm Al₂O₃ particles. The slight increase in hardness of the coating containing the agglomerated 20 nm Al₂O₃ particles occurred in a coating of Al₂O₃ content that was lower than that in the coating that contained the 10 μm reinforcing Al₂O₃ particles. The increased hardness of the MMC coatings that contained the agglomerated 20 nm Al₂O₃ particles and at lower reinforcing particle content was attributed to the increased spreading of the nanoagglomerated particles in the coating, which increased load-sharing and reinforcement capability of the particles. These results suggest that the use of nanoagglomerated, reinforcing hard-phase particles in cold-sprayed MMC coatings may be a more efficient alternative to the use of conventional micronsized reinforcing particles.     

Multiscale Experimental and Numerical Approach to the Powder Particle Shape Effect on Al-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Coating Build-Up

Aluminum (Al) powders with spherical and irregular particle shapes were mixed with two alumina (Al₂O₃) powders with either a spherical or an angular particle shape to achieve high-performance cold-sprayed coatings onto steel. Two effects of the aluminum particle shape were observed. First, coating microstructure observation showed impinging heterogeneity depending on particle shape. Second, particle jet differences depending on particle morphology were shown by velocity maps. From the latter, SEM and XRD, three effects of the alumina particle shape were also shown, i.e., higher in-flight velocity of angular particles, fragmentation of spherical hollow particles and embedding of alumina particles with aluminum. Numerical simulation of particle impacts was developed to study the densification of Al coating due to Al₂O₃ addition through elucidation of Al-Al₂O₃ interaction behavior at the scale of the coating. Al/Al and Al/Al₂O₃ interfaces were investigated using TEM to understand coating strengthening effects due to alumina addition at the scale of the particle. As a whole, Al and Al₂O₃ particle shape effects were claimed to explain coating mechanical properties, e.g., microhardness and coating–substrate bond strength. This study resulted in specifying criteria to help cold spray users in selecting powders for their applications, to meet economic and technical requirements.     

High Pressure Cold Sprayed (HPCS) and Low Pressure Cold Sprayed (LPCS) Coatings Prepared from OFHC Cu Feedstock: Overview from Powder Characteristics to Coating Properties

Cold spraying enables high quality Cu coatings to be deposited for applications where high electrical and/or thermal conductivity is needed. Fully dense Cu coatings can provide an effective corrosion barrier in specific environments. The structure of cold-sprayed Cu coatings is characterized by high deformation which imparts excellent properties. Coating properties depend on powder, the cold spray process and post treatments. First of all, powder characteristics have a strong influence on the formation of pure coatings. Secondly, cold spraying provides dense, adherent, and conductive coatings by using HPCS and LPCS. Furthermore, an addition of Al₂O₃ particles to the Cu powder in LPCS process significantly improves coating properties. Also, heat treatments improve electrical conductivity. This study summarizes optimal characteristics of Cu powder optimized for cold spraying, achieving high coating quality and compares properties of HPCS Cu, LPCS Cu and Cu+Al₂O₃ coatings prepared from the same batch of OFHC Cu powder.     

Formation of Al/(Al<Subscript>13</Subscript>Fe<Subscript>4</Subscript> + Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) Nano-composites via Mechanical Alloying and Friction Stir Processing

Combination of mechanical alloying and friction stir processing was used for the fabrication of Al/(Al₁₃Fe₄ + Al₂O₃) nano-composites. Pre-milled hematite + Al powder mixture was introduced into the stir zone generated on 1050 aluminum alloy sheet by friction stir processing. Uniform and active milled powder mixture reacted with plasticized aluminum to produced Al₁₃Fe₄ + Al₂O₃ particles. Al₁₃Fe₄ intermetallic showed elliptical shape with a typical size of ~ 100 nm, while nano-sized Al₂O₃ exhibited irregular floc-shaped particles that formed clusters with the remnant of iron oxide particles in the fine recrystallized aluminum matrix. As the milling time (1-3 h) of the introduced powder mixture increased, the volume fraction of Al₁₃Fe₄ + Al₂O₃ particles increased in the fabricated composite. The hardness and ultimate tensile strength of the fabricated nano-composites varied from 54.5 to 75 HV and 139 to 159 MPa, respectively; these are much higher than those of the friction stir processed base alloy (33 HV and 97 UTS). The highest hardness and strength were achieved for the nano-composite fabricated using the 3-h milled powder mixture; hard nano-sized reaction products and fine recrystallized grains of Al matrix had major and minor roles on enhancing these properties, respectively.     

Microstructural Characterization and Strengthening-Toughening Mechanism of Plasma-Sprayed Al2O3-Cr2O3 Composite Coatings

In this study, Al₂O₃, Cr₂O₃, and Al₂O₃-Cr₂O₃ coatings were fabricated by plasma spraying. X-ray diffraction was employed to determine the phase composition of powders and coatings. The morphologies and microstructures of the coatings were characterized using electron probe microanalyzer and transmission electron microscopy. Vickers hardness, fracture toughness, and bending strength of the coatings were measured. Al₂O₃-Cr₂O₃ composite coatings show better comprehensive mechanical properties than the individual Al₂O₃ and Cr₂O₃ coatings, which are attributed to the former's larger intersplat adhesion or interlamellar cohesion and lower porosity. Solid solution strengthens the phase interfaces and grain boundaries, which is beneficial to improve the mechanical performance of the composite coatings.     

A facile method for fabrication of nano-structured Ni-Al2O3 graded coatings: Structural characterization

Powder charges of micron-size Ni and Al₂O₃ were utilized to deposit nano-structured Ni-Al₂O₃ composite coatings on an aluminum plate fixed at the top end of a milling vial using a planetary ball mill. Composite coatings were fabricated using powder mixtures with a wide range of Ni/Al₂O₃ mass ratio varying from 1:1 to plain Ni. XRD, SEM and TEM techniques were employed to study the structural characteristics of the coatings. It was found that the composition of the starting mixture strongly affects the Al₂O₃ content and the microstructure of the final coating. Mixtures containing higher contents of Al₂O₃ yield higher volume fractions of the Al₂O₃ particles in the coating. Though Ni-Al₂O₃ composite coatings with about 50% of Al₂O₃ particles were successfully deposited, well-compacted and free of cracks and/or voids coatings included less than 20% (volume fraction) of Al₂O₃ particles which were deposited from powder mixtures with Ni/Al₂O₃ mass ratios of 4:1 or higher. Moreover, mechanical and metallurgical bondings are the main mechanisms of the adhesion of the coating to the Al substrate. Finally, functionally graded composite coatings with noticeable compaction and integrity were produced by deposition of two separate layers under identical coating conditions.     

Effects of the powder manufacturing method on microstructure and wear performance of plasma sprayed alumina-titania coatings

Three Al₂O₃-13wt.% TiO₂ powders, with the same chemical composition but different Al₂O₃-TiO₂ distribution patterns, are plasma sprayed and the resulting coatings are compared in terms of their phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance. It is demonstrated that the degree of mixing of the Al₂O₃ and TiO₂ ingredients in the feed powder has immense impact on the phase composition, microstructure, hardness, crack growth resistance, and abrasive wear performance of the coatings. A high degree of mixing of Al₂O₃ and TiO₂ in the powder state results in more uniform microstructure, higher hardness, higher crack growth resistance, and consequently better abrasive wear resistance of the coating.     

Toughening and strengthening mechanism of plasma sprayed nanostructured Al2O3–13 wt.%TiO2 coatings

The starting materials of Al₂O₃, TiO₂, ZrO₂ and CeO₂ nanoparticles were agglomerated into sprayable feedstock powders and plasma sprayed to form nanostructured coatings. There were net structures and fused structures in plasma sprayed nanostructured Al₂O₃–13 wt.%TiO₂ coatings. The net structures were derived from partially melted feedstock powders and the fused structures were derived from fully melted feedstock powders. The nanostructured Al₂O₃–13 wt.%TiO₂ coatings possessed higher hardness, bonding strength and crack growth resistance than conventional Metco 130 coatings which were mainly composed of lamellar fused structures. The higher toughness and strength of nanostructured Al₂O₃–13 wt.%TiO₂ coatings were mainly related to the obtained net structures.     

Fabrication of Oxide Dispersion Strengthened Bond Coats with Low Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Content

Nanoscale oxide dispersions have long been used to increase the oxidation and wear resistance of alloys used as bond coatings in thermal barrier coatings. Their manufacturing via mechanical alloying is often accompanied by difficulties regarding their particle size, homogeneous distribution of the oxide dispersions inside the powder, involving considerable costs, due to cold welding of the powder during milling. A significant improvement in this process can be achieved by the use of process control agent (PCA) to achieve the critical balance between cold welding and fracturing, thereby enhancing the process efficiency. In this investigation, the influence of the organic additive stearic acid on the manufacturing process of Al₂O₃-doped CoNiCrAlY powder was investigated. Powders were fabricated via mechanical alloying at different milling times and PCA concentrations. The results showed a decrease in particle size, without hindering the homogeneous incorporation of the oxide dispersions. Two powders manufactured with 0.5 and 1.0 wt.% PCA were deposited by high velocity oxygen fuel (HVOF) spraying. Results showed that a higher content of elongated particles in the powder with the higher PCA content led to increased surface roughness, porosity and decreased coating thickness, with areas without embedded oxide particles.     

Fabrication and characterization of Al2O3-metal composite coating on steel plate with nonpressure combustion synthesis

Al2O3-metal composite coatings with different reactants and diluents were fabricated on mild steel plate with nonpressure combustion synthesis process. The coat-ings were characterized by means of X-ray diffraction, scanning electron microscopy, and energy-dispersive spec-trometry, respectively. Thermal shock tests were carried out to determine the bond strength of the coating with the steel substrate. The results indicate that the coating is composed of α-A1203, α-（Fe-Cr） and Al2SiO5 as the main phases. It is found that the coating with the diluents of Al2O3-SiO2 and transition layer of Al2O3-Cr presents the hi.ghest hardness of 2270 HV0.2 and the lowest porosity of 3.93 %. Owing to a metallurgical bond of the coating-to-substrate, the coating exhibits a good thermal shock resistance.     

Splat Formation and Adhesion Mechanisms of Cold Gas-Sprayed Al Coatings on Al2O3 Substrates

The metallization of ceramics by means of cold gas spraying (CGS) has been in the focus of numerous publications in the recent past. However, the bonding mechanism of metallic coatings on non-ductile substrates is still not fully understood. Former investigations of titanium coatings deposited on corundum revealed that a combination of recrystallization induced by adiabatic shear processes and hetero-epitaxial growth might be responsible for the high adhesion strengths of coatings applied on smooth ceramic surfaces. In the present work, the interface formation between CGS aluminum and alumina substrates is examined for different particle sizes and substrate temperatures. Furthermore, the influence of subsequent heat treatment on tensile strength and hardness is investigated. The splat formation of single particles is examined by means of scanning electron microscopy, while a high resolution transmission electron microscope is used to study the Al/Al₂O₃ interface. First results suggest that mechanical interlocking is the primary adhesion mechanism on polycrystalline substrates having the roughness in sub-micrometer range, while the heteroepitaxy between Al and Al₂O₃ can be considered as the main bonding mechanism for single-crystalline sapphire (α-Al₂O₃) substrates with the surface roughness in nanometer range. The heteroepitaxial growth is facilitated by deformation-induced recrystallisation of CGS aluminum.     

Fabrication and performance evaluation of metal bond diamond tools based on aluminothermic reaction

Herein, the fabrication of metal bond diamond tools is proposed by using the Fe₂O₃-Al aluminothermic reaction. Moreover, the influence of sintering temperature and TiH₂- and Si-doping on phase composition and mechanical properties of the reactive sintered bond is investigated. Furthermore, the grinding performance of metal bond diamond tool on ceramic tile is also examined. At 930 °C, the sintered bond is composed of Fe, Al₂O₃, Fe₃O₄ and FeO phases. However, the aluminothermic reaction initiated at 1028.8 °C and resulted in the formation of FeAl₂O₄ and Fe₃Al phases. Moreover, the content of Al₂O₃, Fe₃Al, α-Fe and FeAl₂O₄ phases increased with the increase of sintering temperature. The maximum flexural strength, hardness and relative density are achieved when sintering at 1230 °C. In addition, the dehydrogenation of TiH₂ can impede the formation of FeAl₂O₄ phases and improve the flexural strength, hardness and relative density of the bond. Also, the Si-doping into Fe₂O₃-Al aluminothermic reaction system resulted in Fe₂Al₃Si₃ phase and reduced the content of Al₂O₃ and Fe₃Al phases, leading to higher flexural strength, lower hardness and inferior relative density. In wet grinding, the as-prepared metal bond diamond tool can be used to grind ceramic tiles with lower grinding force and better surface quality than the dry grinding. However, the as-prepared metal bond diamond tool rendered low wear resistance due to the brittle nature of the metal bond.     

过渡材料对等离子喷涂Al2O3梯度陶瓷涂层性能影响

针对Al2O3陶瓷涂层结合强度低、孔隙率高的实际,选择NiAl金属间化合物和金属铜粉作为过渡材料,利用等离子喷涂制备Al2O3梯度陶瓷涂层,并对梯度涂层进行组织形貌观察,测试结合强度和孔隙率.结果表明,梯度涂层的组织表现出宏观的不均匀性和微观连续性的分布特征,NiAl和Cu是金属基体与Al2O3涂层之间过渡层的理想材料,可以有效地提高涂层的结合强度,而Cu-Al2O3梯度涂层又比NiAl-Al2O3梯度涂层结合强度高;梯度涂层的孔隙率远低于双涂层的孔隙率,在Cu-Al2O3梯度涂层中随Al2O3含量的增加,涂层的孔隙率降低,而且孔隙率低于NiAl-Al2O3梯度涂层.