High velocity oxy-fuel spraying (HVOF) being one of thermal spraying techniques, is deployed in many commercial applications mostly for very hard wear and/or corrosion resistant coatings. Supersonic speed of the spraying jet combined with temperatures around 5500 K is utilized to spray most metals, metallic alloys, cermets and superalloys [1][2][3]. Typical HVOF coatings are compact, dense, with very low porosity, low to moderate oxidation level and high bonding strength to substrate [1]. Cermets and superalloys are standardly commercially applied HVOF materials, demanded for superb wear resistance, high hardness, abrasion and erosion resistance and overall great performance in high stress and/or high temperature environments. Proper testing methods are required to evaluate quality of deposited coatings and consequently choose coating with optimized properties for demanded application. There are many testing methods standardly and widely used for evaluating properties of thermally sprayed coatingssuch as superficial indentation, micro and nanoindentation on cross section of the coating, tensile adhesion strength test, many tribological tests, etc. [1]. This article concerns modified TCT (tubular coating tensile test) for testing cohesion strength of the coatings with the use of common tensile test equipment. In this paper, specimen assembly, coating deposition, testing procedure and calculation of results is described. Cohesion strengths of five HVOF commercial coatings: Stellite 6, Hastelloy C 276, Cr3C2-25NiCr, WC 10Co4Cr and NiCrFeSiBC are evaluated.
Atmospheric plasma sprayed (APS) chromia coating provides excellent wear behaviour combined with corrosion resistance even in aggressive environments. In this paper, the influence of variable spraying parameters on the mechanical properties and microstructure of chromia coatings is investigated. Main goal for this optimization is to achieve high wear and abrasive resistance. APS ceramic coatings such as chromia could perform in some industrial applications as well as more commonly used HVOF (high velocity oxygen fuel) cermet coatings, especially in wear and abrasion resistance. In order to be able to compete with hard and wear resistant cermet HVOF coatings, optimization of spraying parameters for this particular chromia powder is required. High throughput cascaded plasma troch is used for the spraying process. Set of five different spraying parameters is used varying only primary plasma gas flow rate and input electrical current with rest of spraying parameters set constant, including carrier gas flow. Critical plasma spraying parameter or CPSP is also utilized in the design stage of experiment. Coatings sprayed on low carbon steel substrate underwent superficial Rockwell indentation, optical microscopy on polished cross section, adhesion strength testing and dry sand abrasion test. No significant correlation between CPSP values and hardness, adhesive strength or abrasion resistance was observed. High input electrical current combined with rather high plasma gas flow provided better abrasion resistance than lower input electrical current and flow rates. Coating performing best in abrasion test also displayed by far the highest adhesive strength to substrate material, so presumably having also the highest cohesion strength between splats may lead to less inter splat decohesion or chipping during dry sand abrasion.
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