Effect of nanostructuring and Al alloying on friction and wear behaviour of thermal sprayed WC–Co coatings

Nanostructured WC–Co and WC–Co–Al coatings, with about 300-μm as-deposited coating thickness, were deposited by high velocity oxy-fuel (HVOF) spraying. Agglomerated nanostructured cermet powders produced by the Mechanomade® process was used for HVOF spraying. Dense and well-adherent coatings with crystal sizes below 30 nm were deposited on stainless steel 304 substrate. Porosity was less than 5% and the bond strength with the substrate was around 60 MPa. Experimental data on friction, wear, and abrasion resistance revealed that nanostructured WC–Co based coatings containing some Al as alloying element, exhibit improved tribological characteristics in comparison to nanostructured and micron-sized WC–Co coatings. This was attributed to a carbide particle distribution within the coating revealed by SEM, the absence of brittle W₂C-like phases revealed by XRD, and the presence of Al at particle/matrix boundaries revealed by TEM.     

Development and trends in HVOF spraying technology

Three actual trends in development of HVOF spraying technology are described and discussed. The trends concern application fields as well as gun and feedstock characteristics. At the example of demountable draw bars it is shown that HVOF sprayed cermet coatings are capable to protect light weight material components subject to dynamical load against wear without detraction of fatigue strength. Personnel and production time savings can be exploited. High deposition efficiency at considerable powder feed rate, high density and negligible oxygen content in corrosion protective iron or nickel based coatings is achieved for spraying with newly developed HVOF guns operating at increased combustion chamber pressures. Also spraying of highly reactive materials like titanium under atmospheric conditions becomes feasible. A major obstacle for industrial use of respective coatings is the lack of adapted characterisation methods that permit to ascertain corrosion protective function. Ultrafine powder feedstock is used in order to reduce overall costs of wear protective cermet coatings due to the possibility to finish coatings by comparatively cheap belt grinding. However, it is shown that the replacement of coatings produced with conventional powder size fractions requires careful consideration of the particular tribological system. While cermet coatings produced with ultrafine powders outperform conventional coatings for sliding wear conditions, their capability to withstand dry abrasive wear stress is poor. The benefits concerning coating production costs may be outweighed by significantly decreased component life time.     

Properties of High Velocity Suspension Flame Sprayed (HVSFS) TiO2 coatings

TiO₂ coatings were manufactured by the High Velocity Suspension Flame Spraying (HVSFS) technique using a nanopowder suspension. Their microstructure, nanohardness, tribological properties and photocatalytic activity were studied and compared to conventional atmospheric plasma sprayed (APS) and HVOF-sprayed TiO₂ coatings manufactured using commercially available feedstock. The HVSFS process leaves a fairly large freedom to adjust coating properties (thickness, porosity, anatase content, hardness, etc…) according to the desired objective. Layers with higher anatase content and higher porosity can be produced to achieve higher photocatalytic efficiency, better than conventional APS and HVOF TiO₂. Alternatively, dense protective layers can be deposited, possessing lower porosity and pore interconnectivity and better wear resistance than as-deposited APS and HVOF layers. In all cases, HVSFS-deposited layers are thinner (20 µm-60 µm) than those which can be obtained by conventional spraying processes.     

Sliding and Rolling Wear Behavior of HVOF-Sprayed Coatings Derived from Conventional, Fine and Nanostructured WC-12Co Powders

Fine structured and nanostructured materials represent a promising class of feedstock for future applications, which has also attracted increasing interest in the thermal spray technology. Within the field of wear protection, the application of fine structured or nanostructured WC-Co powders in the High Velocity Oxy-Fuel flame spraying technique (HVOF) provides novel possibilities for the manufacturing of cermet coatings with improved mechanical and tribological characteristics. In this study the tribological behavior of HVOF sprayed coatings derived from conventional, fine and nanostructured WC-12Co powders under sliding and rolling wear are investigated and the results are compared to C45 steel (Mat.-No. 1.0503). In addition, sliding and rolling wear effects on a microscopic level are scrutinized. It has been shown that under optimized spray conditions the corresponding fine and nanostructured WC-12Co coatings are able to obtain higher wear resistances and lower friction coefficients than the conventional coatings. This can be attributed to several scaling effects of the microstructure and to the phase evolution of the coating, which are discussed.     

New results in High Velocity Suspension Flame Spraying (HVSFS)

High Velocity Suspension Flame Spraying (HVSFS) is a new approach for spraying micron, submicron and nanoparticles with hypersonic speed by feeding a suspension directly into the combustion chamber of a HVOF torch. The aim in mind is to achieve dense coatings with an improved microstructure — probably reaching the nanoscale, from which superior physical properties are expected. Compared to the alternative approach, i.e.. using agglomerated (nano- and micron-sized) powders, direct spraying of suspensions shows much higher flexibility in combining and processing different materials and is far less expensive. Several suspensions consisting of an organic solvent and a solid phase consisting of a micron or a nanopowder have been prepared and HVSFS sprayed. Suspensions containing oxide nanopowders of titanium oxide (n-TiO₂), chromium (III) oxide (n-Cr₂O₃), yttrium stabilized zirconia (n-YSZ) and a n-hydroxyapatite (n-HAP). Furthermore two suspensions containing glass powders (grain size 2-3 μμm after milling) were also sprayed: a Zr-Al-Si containing glass (FOA) and a phosphor containing bioglass (AW). HVSFS coatings were characterized regarding their phase composition and microstructure. Mechanical and tribological properties are compared with standard coatings produced by APS and HVOF.     

TiAlNb intermetallic compound coating prepared by high velocity oxy-fuel spraying

Ti_28.35Al_63.4Nb_8.25 (at.%) intermetallic compound coatings were sprayed onto 316 L stainless steel substrates by HVOF processes using various parameters. By varying the grit blasting pressure between 0.11 and 0.55 MPa, the effects of substrate roughness on the adhesion of TiAlNb thermal sprayed coatings were investigated. The microstructure, porosity and microhardness of the coatings were characterized by SEM, XRD, Image Analysis and Vickers hardness analysis. The tensile adhesion test (TAT) specified by ASTM C 633-79 was used to measure the tensile bonding strength of the coating. The results show that the coatings with substrate roughness of 8.33 μm displayed the best combined strength. TiAlNb coatings had a lamellar microstructure with different spraying parameters. The porosity, bonding strength, microhardness of coatings were assessed in relation to the spraying processes. The thickness of bond coat on the bond strength of coatings was also discussed.     

Influence of feedstock powder characteristics and spray processes on microstructure and properties of WC-(W,Cr)2C-Ni hardmetal coatings

The composition WC-(W,Cr)₂C-Ni (commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni and WC-NiCr) is unique among the WC-based materials used for the preparation of thermally sprayed hardmetal coatings. These coatings show a significantly higher oxidation resistance and high-temperature sliding wear resistance than WC-Co and WC-CoCr coatings do. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase-binder metal composite as it is composed of two hard phases: WC and (W,Cr)₂C. Surprisingly this composition has been poorly investigated in the past.In this paper coating microstructures and properties obtained from five commercial feedstock powders of different origins using two different liquid-fuelled high velocity oxy-fuel (HVOF) systems (K2 and JP-5000) were investigated. Additional experiments were performed with one powder using atmospheric and vacuum plasma spraying (APS and VPS, respectively). The microstructures and phase compositions of the powders and the coatings were studied. Focus was on the appearance, composition and distribution of the (W,Cr)₂C phase which might form or might change its Cr/W ratio during the spray process. The composition of the (W,Cr)₂C phase was estimated from the lattice parameters. Hardness HV0.3 was measured for all coatings. The density, Young's modulus and abrasion wear resistance of HVOF-sprayed coatings were studied.     

Influence of the metallic matrix ratio on the wear resistance (dry and slurry abrasion) of plasma sprayed cermet (chromia/stainless steel) coatings

This work is related to the manufacturing and tribological testing of plasma sprayed cermet coatings of chromium oxide and stainless steel in order to obtain wear resistant coatings to dry and slurry abrasion. Raw materials were fused and crushed powders of chromium oxide (Cr₂O₃) with a particle size ranging from 20 to 45 μm and gas atomized stainless steel (iron base with 17 wt.% of Cr and 12 wt.% of Ni) with a particle size distribution between 20 and 53 μm. Both powders were simultaneously injected with two separated injectors in a direct current (DC) plasma jet (Ar-H₂ (25 vol.%) at atmospheric pressure (APS).The influence, on the coating micro-structural and tribological properties, of various stainless steel weight percentages in chromium oxide has been studied. All coatings exhibited a lamellar structure with a random distribution of the two materials. The effect of the percentage of stainless steel on the microstructure of the coating, studied by scanning electron microscopy (SEM), has shown that increasing the stainless steel percentage increased the coating cohesion. The increase of Cr₂O₃ in the coatings resulted in higher hardness and in lower weight losses during wear tests in dry abrasion. The study has also shown that the optimum stainless steel percentages in coatings were not identical to reach their maximum resistance to slurry or dry abrasion.     

Microstructural and Tribological Investigation of High-Velocity Suspension Flame Sprayed (HVSFS) Al2O3 Coatings

Al₂O₃ coatings were manufactured by the high-velocity suspension flame spraying (HVSFS) technique using a nanopowder suspension. Their structural and microstructural characteristics, micromechanical behavior, and tribological properties were studied and compared to conventional atmospheric plasma sprayed and high-velocity oxygen-fuel-sprayed Al₂O₃ coatings manufactured using commercially available feedstock. The HVSFS process enables near full melting of the nanopowder particles, resulting in very small and well flattened lamellae (thickness range 100 nm to 1 μm), almost free of transverse microcracking, with very few unmelted inclusions. Thus, porosity is much lower and pores are smaller than in conventional coatings. Moreover, few interlamellar or intralamellar cracks exist, resulting in reduced pore interconnectivity (evaluated by electrochemical impedance spectroscopy). Such strong interlamellar cohesion favors much better dry sliding wear resistance at room temperature and at 400 °C.     

Mechanical and Thermal Transport Properties of Suspension Thermal-Sprayed Alumina-Zirconia Composite Coatings

Micro-laminates and nanocomposites of Al₂O₃ and ZrO₂ can potentially exhibit higher hardness and fracture toughness and lower thermal conductivity than alumina or zirconia alone. The potential of these improvements for abrasion protection and thermal barrier coatings is generating considerable interest in developing techniques for producing these functional coatings with optimized microstructures. Al₂O₃-ZrO₂ composite coatings were deposited by suspension thermal spraying (APS and HVOF) of submicron feedstock powders. The liquid carrier employed in this approach allows for controlled injection of much finer particles than in conventional thermal spraying, leading to unique and novel fine-scaled microstructures. The suspensions were injected internally using a Mettech Axial III plasma torch and a Sulzer-Metco DJ-2700 HVOF gun. The different spray processes induced a variety of structures ranging from finely segregated ceramic laminates to highly alloyed amorphous composites. Mechanisms leading to these structures are related to the feedstock size and in-flight particle states upon their impact. Mechanical and thermal transport properties of the coatings were compared. Compositionally segregated crystalline coatings, obtained by plasma spraying, showed the highest hardness of up to 1125 VHN_{3 N}, as well as the highest abrasion wear resistance (following ASTM G65). The HVOF coating exhibited the highest erosion wear resistance (following ASTM G75), which was related to the toughening effect of small dispersed zirconia particles in the alumina-zirconia-alloyed matrix. This microstructure also exhibited the lowest thermal diffusivity, which is explained by the amorphous phase content and limited particle bonding, generating local thermal resistances within the structure.     

Development of multimaterial coatings by cold spray and gas detonation spraying

The basic objective is the development of multifunctional multimaterial protective coatings using cold spraying (CS) and computer controlled detonation spraying (CCDS).As far as CS is concerned, the separate injection of each powder into different zones of the carrier gas stream is applied. Cu-Al, Cu-SiC, Al-Al₂O₃, Cu-Al₂O₃, Al-SiC, Al-Ti and Ti-SiC coatings are successfully sprayed. As to CCDS, powders are sprayed with a recently developed apparatus that is characterized by a high-precision gas supply system and a fine-dosed twin powder feeding system. Computer control provides a flexible programmed readjustment of the detonation gases energy impact on powder thus allowing selecting the optimal for each component spraying parameters to form composite and multilayered coatings. Several powders are sprayed to obtain composite coatings, specifically, among others, WC-Co-Cr + Al₂O₃, Cu + Al₂O₃, and Al₂O₃ + ZrO₂.     

Ti2AlC coatings deposited by High Velocity Oxy-Fuel spraying

High Velocity Oxy-Fuel has been utilized to spray coatings from Ti₂AlC (MAXTHAL 211^®) powders. X-ray diffraction showed that the coatings consist predominantly of Ti₂AlC with inclusions of the phases Ti₃AlC₂, TiC, and Al-Ti alloys. The fraction of Ti₂AlC in coatings sprayed with a powder size of 38 μm was found to increase with decreasing power of the spraying flame as controlled by the total gas flow of H₂ and O₂. A more coarse powder (56 μm) is less sensitive to the total gas flow and retains higher volume fraction of MAX-phase in the coatings, however, at the expense of increasing porosity. X-ray pole figure measurements showed a preferred crystal orientation in the coatings with the Ti₂AlC (000l) planes aligned to the substrate surface. Bending tests show a good adhesion to stainless steel substrates and indentation yields a hardness of 3-5 GPa for the coatings sprayed with a powder size of 38 μm.     

Effect of the suspension composition on the microstructural properties of high velocity suspension flame sprayed (HVSFS) Al2O3 coatings

Seven different Al₂O₃-based suspensions were prepared by dispersing two nano-sized Al₂O₃ powders (having analogous size distribution and chemical composition but different surface chemistry), one micron-sized powder and their mixtures in a water + isopropanol solution. High velocity suspension flame sprayed (HVSFS) coatings were deposited using these suspensions as feedstock and adopting two different sets of spray parameters.The characteristics of the suspension, particularly its agglomeration behaviour, have a significant influence on the coating deposition mechanism and, hence, on its properties (microstructure, hardness, elastic modulus). Dense and very smooth (Ra ~ 1.3 μm) coatings, consisting of well-flattened lamellae having a homogeneous size distribution, are obtained when micron-sized (~ 1-2 μm) powders with low tendency to agglomeration are employed. Spray parameters favouring the break-up of the few agglomerates present in the suspension enhance the deposition efficiency (up to > 50%), as no particle or agglomerate larger than ~ 2.5 μm can be fully melted. Nano-sized powders, by contrast, generally form stronger agglomerates, which cannot be significantly disrupted by adjusting the spray parameters. If the chosen nanopowder forms small agglomerates (up to a few microns), the deposition efficiency is satisfactory and the coating porosity is limited, although the lamellae generally have a wider size distribution, so that roughness is somewhat higher. If the nanopowder forms large agglomerates (on account of its surface chemistry), poor deposition efficiencies and porous layers are obtained.Although suspensions containing the pure micron-sized powder produce the densest coatings, the highest deposition efficiency (~ 70%) is obtained by suitable mixtures of micron- and nano-sized powders, on account of synergistic effects.     

Microstructure and Properties of HVOF-Sprayed WC-(W,Cr)<Subscript>2</Subscript>C-Ni Coatings

The composition WC-(W,Cr)₂C-Ni is one of the standard compositions used for the preparation of thermally sprayed coatings by high velocity oxy-fuel (HVOF) spraying. Surprisingly, this composition has been poorly investigated in the past. Frequent use of commercial designations WC-‘CrC’-Ni, WC-Cr₃C₂-Ni, and WC-NiCr indicates the insufficient knowledge about the phase compositions of these powders and coatings. The properties of these coatings differ significantly from those of WC-Co and WC-CoCr coatings. In this paper, the results of different series of experiments conducted on HVOF-sprayed WC-(W,Cr)₂C-Ni coatings are compiled and their specific benefits pointed out. The focus of this study is on the analysis of the microstructures and phase compositions of the feedstock powders and coatings. Unlike WC-Co and Cr₃C₂-NiCr, WC-(W,Cr)₂C-Ni is not a simple binary hard phase—binder metal composite. The phase (W,Cr)₂C with unknown physical and mechanical properties appears as a second hard phase, which is inhomogeneously distributed in the feedstock powders and coatings. As examples of coating properties, the oxidation resistance and dry sliding wear properties are compared with those of WC-10%Co-4%Cr coatings.     

Residual stresses in HVOF-sprayed ceramic coatings

In this paper, the residual stress state of thermally sprayed ceramic coatings was examined by combining different experimental and analytical techniques, in order to provide a thorough characterisation of through-thickness stress profiles and a cross-verification of results. HVOF-sprayed ceramics, manufactured using commercial and nanostructured Al₂O₃ powders and commercial Cr₂O₃ powders, and atmospheric plasma-sprayed (APS) ceramics, manufactured using commercial Al₂O₃ and Cr₂O₃ powders, were investigated.The near-surface stress was measured by X-ray diffraction. The through-thickness profile and the intrinsic quenching stress were analytically computed by the Tsui-Clyne iterative model, using the X-ray measurement result as input, and results were validated by the substrate chemical removal method. Further verification was achieved by applying the in-situ curvature technique to the deposition of HVOF-sprayed Al₂O₃ coating.HVOF-sprayed Al₂O₃ coatings deposited using both conventional and nanostructured powders feature a similar, almost equibiaxial tensile stress on the top surface (116.5 MPa and 136.5 MPa, respectively) and a moderate through-thickness gradient (about 12 MPa and 20 MPa, respectively). Their intrinsic quenching stresses were analytically estimated to be 184 MPa and 205 MPa, respectively. APS Al₂O₃ possesses higher top surface stress (220 MPa) and quenching stress (311 MPa). However, it shows a less pronounced stress gradient (≈ 3 MPa) than HVOF-sprayed Al₂O₃-based coatings, because cracks, pores and weak lamella boundaries in the APS coating can accommodate the deformations induced by the bending moments arising both during coating deposition and during cooling.The model-derived quenching stress of the conventional HVOF Al₂O₃ coating was validated by the in-situ curvature measurement technique.Cr₂O₃-based coatings are significantly different. They display a lower residual stress in the near-surface region: 20 MPa in the APS coating, 27.5 MPa in the HVOF one. The HVOF coating also exhibits a very large stress gradient of ≈ 77 MPa. Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.     

Influence of the HVOF Gas Composition on the Thermal Spraying of WC-Co Submicron Powders (−8 + 1 μm) to Produce Superfine Structured Cermet Coatings

Thermal spraying technology represents a novel and promising approach to protect forming tools with complex surfaces and highest shape accuracy against abrasive wear and galling. However, due to high or nonuniform layer thicknesses or inappropriate surface roughness conventional coarse-structured coatings are not suitable to achieve this aim. The application of novel submicron or nanoscaled feedstock materials in the thermal spray process can provide the deposition of cermet coatings with significantly improved characteristics and is recently of great interest in science and industry. In this collaborative study, the feeding and HVOF spraying of WC-Co submicron powders (?8 + 1 μm) have been investigated to manufacture superfine structured, wear resistant, near-net-shape coatings with improved macroscopic properties and smooth surfaces. The influences of varying HVOF gas compositions on the spray process and the coating properties have been analyzed.     

Mechanical and tribological performance of Al2O3-TiO2 coatings elaborated by flame and plasma spraying

Mechanical properties and wear rates of Al₂O₃-13 wt.% TiO₂ (AT-13) and Al₂O₃-43 wt.% TiO₂ (AT-43) coatings obtained by flame and atmospheric plasma spraying were studied. The feed stock was either ceramic cords or powders. Results show that the wear resistance of AT-13 coatings is higher than that of AT-43 and it seems that the effect of hardness on wear resistance is more important than that of toughness. Additionally, it was established that, according to conditions used to elaborate coatings and the sliding tribological test chosen, spray processes do not seem to have an important effect on the wear resistance of these coatings.     

Isothermal oxidation resistance comparison between air plasma sprayed, vacuum plasma sprayed and high velocity oxygen fuel sprayed CoNiCrAlY bond coats

Commercial CoNiCrAlY powders with the same chemical composition were sprayed by vacuum plasma spraying (VPS), air plasma spraying (APS) and high velocity oxygen fuel (HVOF) onto Hastelloy X superalloy substrates obtaining coatings of comparable thickness. After coating, samples were maintained at 1273 K in air for different periods up to 3000 h. Morphological, microstructural and compositional analyses were performed in order to assess the high temperature oxidation resistance provided by the different spraying systems. HVOF technique provided bond coats with higher oxidation resistance compared to APS and VPS.     

Lightweight TiC/Ti wear-resistant coatings for lightweight structural applications

Lightweight coatings based on titanium and titanium carbides produced by plasma spraying can be used to improve and modify the tribomechanical properties of aerospace structural materials. Although plasma-sprayed WC/Co coatings have been applied with success in many cases, such as primary wear-re-sistant materials, their high densities preclude their use in applications that mandate reduction in weight. In the present investigation, the sliding wear resistance of plasma-sprayed, metal-bonded TiC coatings on AI 7075 substrates was studied. Coatings containing 50, 70, and 90 vol% TiC in a Ti matrix produced from physically blended powders of Ti and TiC were compared. Metallographie evaluations showed that dense coatings with good bonding to AI 7075 substrates can be obtained. Coatings from commercial pu-rity (CP) Ti powders sprayed in air under atmospheric conditions, however, indicated considerable oxi-dation of the particles. Under dry sliding conditions, the coefficient of friction (COF) values of the Ti/TiC containing/Al 7075 substrate system were lower than high-velocity oxygen fuel (HVOF) sprayed 75% Cr₃C₂/25%NiCr coatings on steel and were comparable to coatings of WC/Co. Vacuum plasma-sprayed TiC/Ti coatings with 90 vol% TiC also exhibited better wear resistance than HVOF sprayed 75%Cr₃C₂/25%NiCr.     

Microstructure and magnetic properties of HVOF thermally sprayed Fe75Si15B10 coatings

Partially amorphous Fe₇₅Si₁₅B₁₀ coatings were prepared from nanostructured feedstock powders by using high velocity oxy-fuel spraying. Scanning electron microscopy, X-ray diffraction, Vickers indenter and magnetic measurements were used to investigate microstructural, structural, microhardness and magnetic properties of the coatings. The Rietveld refinement of the X-ray diffraction patterns reveals the presence of an amorphous phase, nanocrystalline α-Fe(Si,B) structure having a lattice parameter close to 0.2841 nm and an average crystallite size of about 78-83 nm in addition to small amounts of Fe₃O₄ oxide (104 nm) and Fe₂B boride (151 nm), which disappear completely with increasing coating thickness. Coercivity and microhardness values are 15.5 Oe and 478 Hv, respectively, for 84 μm thickness.