Frequent reporting of microhardness data for thermal spray coatings testifies to the widespread use of this technique for coatings characterization. However, inadequate reporting of microhardness procedures makes comparisons between published coatings hardness statistics difficult and it appears that both microhardness in general and its significance to characterizing thermal spray coatings in particular, are poorly understood. This paper demonstrates that though microindentation technique is a useful laboratory procedure that can be used for coatings optimization, research and quality control purposes, poor understanding often leads to worthless data and thus to erroneous conclusions. A high quality WC-12%Co coating supplied by Sulzer Metco was hardness tested on both the polished cross-section and plane surface of the coating. Contributions to the variance in results obtained and sources of significant errors are discussed and conclusions are drawn regarding the methodology and suitability of hardness testing for characterizing thermal spray coatings. The limits in repeatability and reproducibility of Vickers microhardness data for hard metal thermal spray coatings are discussed. The necessity for rigorous statistical procedures of data analysis is demonstrated. It is suggested that the technique is inherently unsuitable for characterizing hard thermal spray coatings due to poor intrinsic reproducibility.

A typical feature of plasma sprayed zirconia-based coatings is that their depostion efficiencies are low. Thus, from the economic view of reducing cost, how to improve the depostion efficiency of plasma sprayed zirconia-based coatings is very important for practical application. In this paper, spray-dried nanostructured powders and conventional powders were used as feedstocks to deposit nanostructured and conventional zirconia coatings by atmospheric plasma spraying(APS). Deposition efficiencies for both nanostructured and conventional zirconia coatings deposited under different spraying parameters were comparatively measured. The obtained results revealed that the spray-dried nanostructured powders possessed higher deposition efficiency than that of conventional powders. In addition, it was found that spraying parameters had strongly influence on microstructure and microhardness of zirconia coatings. Therefore, in order to obtain high quality zirconia coating, proper spraying parameters must be considered.

High temperature titanium alloys are considered as good candidate materials for many aerospace applications. In order to increase the usable temperatures and oxidation resistance of titanium alloys, plasma spraying thermal barrier coatings on the titanium alloys is considered as an effective method. The effect of plasma spraying process on microstructure and microhardness of the titanium alloy was investigated by scanning electron microscope, energy dispersion analytical X-ray spectroscopy and microhardness test. The results show that the microstructure of the titanium alloy inside the substrate keeps unchanged after plasma spraying, and no interaction and atomic diffusion happen evidently at the bond coat/substrate interface. However there exists a thin layer of plastic deformation zone in the substrate beneath the bond coat/substrate interface after plasma spraying. The residual stresses are induced inside the titanium alloy due to the thermal expansion mismatch and the temperature gradient inside the substrate during plasma spraying, and lead to generating microcracks in the surface beneath the bond coat/substrate interface and the increase of microhardness in the substrate.

In the present study, a nanostructured FeAl coating was prepared by cold spraying of ball milled powder. Annealing treatment was applied to the coating to investigate its effect on the phase structure, grain size and microhardness of the cold-sprayed nanostructured FeAl coating. The results showed that the FeAl phase was kept unchangeable when the coating annealed at the temperature above 500°C. Annealing temperature significantly influenced the microstructure and microhardness of cold-sprayed FeAl coating. With raising annealing temperature, the lamellar structure in the as-sprayed coating disappeared and a dense coating microstructure with fully bonding of deposited particles at their interfaces was achieved after annealing at 950°C. Nanograin growth of the FeAl phase occurred at an annealing temperature higher than 800°C. The microhardness of cold-sprayed FeAl coating remained about 400 Hv 0.1 at the annealing temperature below 800°C and decreased to 300 Hv 0.1 at 1100°C.

Thermally sprayed coatings are applied to various substrates for the purpose of providing a wear-resistant, corrosion-resistant, or heat barrier surface. The materials used to coat a substrate will vary in hardness with respect to each other and with respect to the substrate. Denser coatings produce more reliable hardness data than less dense coatings due to the absence of pores and other inconsistencies in the coating. Variations in coatings will cause preparation problems such as pullout, edge rounding, or coating-to-substrate relief which effects the reliability of hardness determinations. The repeatability of hardness data is directly related to the density of the coating. This paper deals with the effect of coating characteristics (density) on the reliability of manual and automated hardness measurements.

Thermal spray coatings of new MoB/CoCr cermets were developed. The mechanical behavior of HVOF-sprayed MoB/CoCr novel composite coatings was evaluated via Vickers microhardness. Microstructure of the coatings on 316L stainless steel substrates, as well as powders, were studied with optical microscopy, scanning electron microscopy (SEM), X-ray diffraction (XRD). X-ray microanalysis of the coatings was carried out using energy dispersive X-ray spectrometer (EDS) attached to the SEM. The distributions of microhardness values of the coatings were analyzed via Weibull statistic. Weibull analysis revealed a bimodal distribution of Vickers microhardness values. Such distribution was attributed to the presence of melted and unmelted phases in the resultant coating produced from the microstructured powder feedstock. The excellent mechanical properties of the coating are due to the MoB/CoCr powder, which results in the formation of complex ternary transition metal boride hard particles that exhibit exceptional mechanical properties.

Thermal spraying may be a promising approach for the deposition of NiAl-Al 2 O 3 composite coatings. To understand the intrinsic mechanical property of NiAl-Al 2 O 3 composite, dense NiAl-Al 2 O 3 composites were prepared by plasma activated sintering using the ball milled Ni/Al- Al 2 O 3 composite powders contain both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids homogeneously distributed in the NiAl matrix phase. The angular morphology of the Al 2 O 3 particles in composite powder was changed to spherical or near spherical morphology after plasma activated sintering. Compared to the NiAl-Al 2 O 3 composites with low Al 2 O 3 content, NiAl- 60vol.%Al 2 O 3 composite consisted of less fine submicron-sized Al 2 O 3 dispersoids. Melting induced mechanism was proposed to explain the spheroidization and the change of Al 2 O 3 particle size distribution. The hardness of the NiAl-Al 2 O 3 composites with both micro-sized Al 2 O 3 strengthening particles and submicron-sized Al 2 O 3 dispersoids increased with the increase of Al 2 O 3 content. The relation was employed to theoretically estimate the hardness using volume fractions of components and corresponding hardness. The estimated hardness was compared with the observed ones. It was found that bimodal sized particles reinforcement results in a higher hardness than the theoretical value, which could be attributed to the dispersion hardening effect.

Cu coating was deposited by microplasma spraying system under a low power of 2.8 to 4.2kW. The effects of the main processing parameters including plasma arc power, operating gas flow and spray distance on particle velocity during spraying, and the microstructure and properties of the coating were investigated. The coating microstructure was examined with optical microscopy. The coating properties were characterized by cross sectional microhardness. The particle velocity during in-flight was examined using a particle velocity/temperature measurement system based on thermal radiation. The experiment results showed that particle velocity was increased with the increase in operating gas flow, and was not influenced significantly by plasma arc power and spray distance. Moreover, the microhardness of the coating was increased with the increase in arc power and with the decrease in spray distance. The operating gas flow showed no significant influence on the microhardness of the coating. The analysis suggested that the microhardness of the coating is influenced significantly by particle temperature. The comparison showed that the microhardness of the Cu coating deposited by microplasma spray is comparable to that of the coating deposited by conventional plasma spray system at power level of 30kW.

Two kinds of thermal barrier coatings with NiCoCrAlY bond coatings (BCs) deposited by electron beam-physical vapor deposition (EB-PVD) and high velocity oxy-fuel thermal spraying (HVOF), respectively, as well as their top 8wt.%Y 2 O 3 -ZrO 2 (YSZ) ceramic layers deposited in one batch by EB-PVD were prepared on near-α titanium alloys. The field emission scanning electronic microscopy and microhardness indentation are used in comparatively study of microstructures, microhardness of samples. Cracking modes and crack characteristics in TBCs are investigated after thermal cycling in atmosphere, along with the discussion of roles of residual stresses, bonding strengths and mechanical properties of bond coatings in different failure extents. It is found that morphologies of BCs deposited by different methods (EB-PVD and HVOF) result in the different microstructures and microhardness of their upper YSZ. The denser and more homogeneous BC prepared by EB-PVD leads to the YSZ with finer and denser columnar clusters and higher microhardness, and the inhomogeneous and porous latter results in the upper YSZ with coarser and loosely bonded columnar grains and lower microhardness, and the TBC with BC deposited by EB-PVD is more protective, which is synthetically induced by residual stresses, bonding strengths and mechanical properties of bond coatings.

Although research has been reported on pre and post processes for cold spraying, quantitative results are lacking. This study aims to quantify the effect of pre-coat grit blasting and post-spray stress relief and annealing treatments on cold-spray coating microhardness, bond strength, and microstructure. It was found that stress relief treatments reduce hardness, but have little effect on adhesion. Annealing also reduces hardness, but is shown to significantly improve adhesive bond strength. Grit blasting, on the other hand, was found to have a detrimental effect on tensile adhesion strength with little impact on coating microhardness.

Results of macro- and microhardness investigations of newly developed coatings are presented in this paper. The coatings were produced with flame- and arc spraying technique. The coating structure and hardness is significantly influenced by the spraying method and spraying parameters. It will be shown that properties for flame sprayed coatings are more effected by parameter variations than for arc sprayed coatings. A higher hardness, a greater density and a better particle cohesion can be obtained by using the arc spraying method.

TiO 2 and TiO 2 -Al composites coatings were prepared by plasma spraying using a reconstituted nanosized feedstock via a spray drying method. Effects of various spray conditions on the microstructure, porosity, microhardness and wear resistance related to the mechanical performance of coatings were evaluated. The coatings sprayed at relatively low plasma power were composed of two distinct microstructures of well defined lamellar structure, similar to microstructure of conventional plasma sprayed coatings typically observed from fully melted particles, and embedded nano or sub-micron particles originated from partial/non-molten particles of feedstock materials. The fraction of partial/non-molten particles in coating layers was increased by Al additive. Such a characteristic of blended microstructure of coatings was clearly confirmed from a bimodal distribution of microhardness described by Weibull plots. The optimized addition of Al into TiO 2 improved mechanical properties such as microhardness and wear resistance.

The aim of this study was to investigate the microstructure (including surface roughness, microstructure and microhardness) of cold sprayed copper coatings on 2017 Al alloy, before and after annealing at 350°C for 1h. It seemed that keeping constant the powder flow rate and increasing the gas pressure from 2.0 to 2.5 MPa, the surface roughness of the coating tended to decrease, while the thickness of the coating tended to increase. Also, it seemed that keeping constant the pressure and increasing the powder flow rate from 22 to 130 g/min, the surface roughness and the thickness of the coating tended to increase. The microstructural study of the coatings after etching revealed particle interfaces and in some cases grain boundaries. Twins were observed in some of the coatings. Negligible porosity, absence of cracks and good adhesion of the coatings to the substrate were observed. The microhardness of the coatings varied between 95 HV 0.3 and 150 HV 0.3 for the different employed cold sprayed conditions. After coatings’ annealing the microhardness of all the coatings (cold sprayed and HVOF sprayed) decreased. The aforementioned results were compared with those of HVOF coating.

In this study, WC-12Co powders with multimodal-sized WC particles were prepared by ball milling and deposited onto stainless steel substrates by cold spraying. Microhardness and fracture toughness were measured on cross-sections by the indentation approach. Coatings produced from powders with small carbide particles, resulting from long milling times, were found to exhibit high microhardness and high fracture toughness. Deposits sprayed with high porosity powders produced by cold compaction had low microhardness, but high fracture toughness.

Many studies have investigated methods to reduce cavitation damage in hydraulic turbines and reduce residual stresses after coating deposition. In this work, a cobalt stainless steel was applied by arc thermal spraying. The influence of air pressure deposition and plasma remelting on the microstructure, oxide volume fraction, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties of the AS895HY cobalt stainless steel coatings were investigated by x-ray diffraction, optical microscopy, scanning electron microscopy, microhardness testing, and ultrasonic cavitation testing (ASTM G32-93). The increase in air pressure, 280 to 550 kPa, modified the oxide fraction from 10.9±1.8% to 24.1±2.8% in the samples. The mass loss results in the cavitation tests were 13.8, 19.2, and 15.0 mg/h for the samples with 280, 410, and 550 kPa, respectively. The remelting of the sprayed coatings eliminated the oxides and porosity. Austenite formation was observed in the two remelted layers with decreases in microhardness; for the first layer, this occurred because of the AWS309L substrate dilution. The PTA remelting reduces the mass loss rate to 0.497 mg/h, with 8.02 hours incubation period. Phase transformations were observed in the remelted coating, but not verified in the arc thermal sprayed coatings.

The deposition of cavitation-resistant materials coatings in turbine blades is an important way to reduce cavitation damage. Fe-Cr-Mn-Si is a cavitation-resistant steel with many deoxidation elements, which can be important for arc thermal spraying materials. The influence of air pressure, arc tension, and chemical composition on the microstructure, area fraction oxide, porosity, microhardness, and cavitation resistance were studied. Microstructures and properties were investigated by XRD, optical and electronic microscopy, microhardness testing, and ultrasonic cavitation testing per ASTM G32-93. Chromium addition promotes an increase in area fraction oxide, and reduces the porosity, changing the microhardness. An increase in air pressure raised the oxide fraction in the SMA_A and 2 alloys. The SMA_A mass loss rates were 31.8, 25.8, and 37.2 mg/h, respectively, for the samples with 280, 410, and 550 kPa of air pressure. For the SMA_3 samples, the increase in the arc voltage reduces the oxide fraction, changing the mass loss rate to 43.8, 32.4, and 29.4 mg/h for 25, 30, and 35 V, respectively. Phase transformations occurred in the arc thermal spray, for all coatings, during cavitation tests. The SEM analysis verified that the mass loss in arc thermal spray coatings occurred because of the oxide fracture and delamination of the splats.

In this study, Al-Sn binary alloy coatings were prepared with Al-5wt.%Sn (Al-5Sn) and Al-10wt.%Sn (Al-10Sn) gas atomized powders by low pressure and high pressure cold spray process. The microstructure and microhardness of the coatings were characterized. The as-sprayed coating were heat treated at 150, 200, 250 and 300 °C for 1 hour, respectively. The effect of heat treatment on microstructure, microhardness and content of Sn phase of the coatings were investigated. The bonding strength of as-sprayed and heat treated Al-Sn coatings were also studied. The results show that the dense and well-bonded Al-10Sn coating can be deposited by low pressure with helium gas while Al-5Sn coating by high pressure cold spray with nitrogen gas. The content of Sn for both Al-5Sn and Al- 10Sn in as-sprayed coatings are consistent with that in feed stock powders. The coarsening and/or migration of Sn phase in both coatings were observed when the annealing temperature exceeds 200 °C. Furthermore, the microhardness of the coatings decreased significantly under the annealing temperature of 250 °C. EDAX analysis shows that the heat treatment has no significant effect on content of Sn phase in Al-5Sn coatings. Bonding strength of as-sprayed Al-10Sn coating is slightly higher than that of Al-5Sn coating. Annealing under 200 °C can increase the bonding strength of Al-5Sn coatings.

A crystalline hydroxyapatite (HA) coating was deposited using gas tunnel type plasma spraying at different spraying distances. In this study, the microstructure, adhesion and microhardness of the resulting coatings were investigated to clarify the effect of the spraying distance on the features of sprayed coatings. The results showed that the spraying distance greatly affects the microhardness, adhesion, and crystallinity of the sprayed HA coatings. Microhardness value was increased as the spraying distance was decreased, mainly due to the formation of dense coating with lower porosity. Adhesion was increased, as the spraying distance was decreased due to a good mechanical bonding between the first apatite layer and the substrate. X-ray diffraction patterns of HA coatings showed the formation of high crystalline apatite coating at short spraying distance.

Characterization of flame sprayed and furnace fused NiCrBSiC alloy coatings with two different carbon contents and 15~45 wt.% WC-Co addition is described in terms of microstructure, microhardness, and differential thermal analysis. Microstructural development of these coatings before and after fusing treatment is discussed to identify the precipitates in the coatings. Optimum fusing conditions (time and temperature) for wear testing sample are investigated in terms of microhardness and porosity of the coatings. Wear performance of these coatings is also investigated by two-body and three-body abrasive and dry sliding wear experiments. Optimum tungsten carbide content of the coatings is also selected to improve wear performance and thus enhance the service life of the process roll for cold rolling steel plant. Finally, microstructure and microhardness of the furnace and induction fused coatings are compared with emphasis on the interface between the coating and the steel substrate.

This study investigates structure and properties of the plasma sprayed coating from Fe–30Cr-0.6C steel that was then finished using electromechanical treatment (EMT). As a consequence of the coating thus formed, surface microhardness under a 200 gf load increased from 3.4 ± 0.4 GPa to 5.2 ± 0.4 GPa and number of pores in the coating structure reduced from 8.0 ± 1.5% to 2.0 ± 0.5%. The results of the microstructural studies showed fundamental changes of the plasma sprayed coating treated by the EMT with the formation of nanostructured crystalline phases. The obtained results revealed the high potential of the EMT for posttreatment of plasma sprayed coatings.