HVOF coatings as an alternative to hard chrome for pistons and valves

doi:10.1016/j.wear.2005.12.005

Wear

Volume 261, Issues 5–6, 20 September 2006, Pages 477-484

https://doi.org/10.1016/j.wear.2005.12.005 Get rights and content

Abstract

Automotive manufacturers have specified chromium plating for decades because of its appearance, wear and corrosion resistance, however chromium plating cause effects on human health because of the use of substances in the galvanic process whose toxicological features have not always been recognised. The improvements of the high-velocity oxy-fuel thermal spray process allow the chromium coating replacement with a comparable or superior surfaces and more environment friendly. The present study describes and compare the mechanical and tribological properties of HVOF CrC75 (NiCr20) 25 coatings sprayed from three different agglomerated feedstock powders with various powder size distributions. These results have been compared with conventional hard chromium plating. The objective of the present work is applying these new HVOF coatings in piston rings and valve stems applications. Furthermore, the studied HVOF coatings are produced with fine-powders in order to avoid the blasting and regrinding operations necessary when plasma spray coatings are used. The coating microstructures were characterised by BSE–SEM microscopy. Differences in roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a pin on disc tribometer under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the CrC–NiCr coating, obtained with the lowest feedstock powder size, presented the best wear resistance under all the studied conditions. The Fine CrC–NiCr coatings have demonstrated superior performance to hard chrome with regard to mechanical and tribological properties, and they can be proposed as an alternative to hard chrome coatings.

Introduction

Functional hard chrome plating is a critical process associated with manufacturing and maintenance operations on aircraft, vehicles and ships, both in civilian and military sectors. Hard chrome electroplating is commercially used to produce wear-resistant coatings, but the plating bath contains hexavalent chromium, which has adverse health and environmental effects. For this reason, the use of hexavalent chromium will be limited. As a results, the European Parliament and the Council on end-of-life vehicles have published (21/10/2000) in the Official Journal of the European Union the Directive 2000/53/EC which expressly prohibits the use of lead, cadmium, hexavalent chromium and mercury in all vehicles put on the market after 1.07.2003 (passenger cars and commercial vehicles up to 3.5 tonnes). The total permissible amount of hexavalent chromium is limited to 2 g per vehicle. This directive must be implemented as national law in the Member States of the European Union. Nevertheless, the implantation of this directive does not limit the possible utilization of other galvanic coatings, like nickel or tin, which are less environmentally damaging.

The types of coatings that are most widely viewed as being capable of replacing hard chrome plating are the thermal spray technologies, especially high-velocity oxy-fuel (HVOF) thermal spraying. With this process, the coating material, in powder form, is fed into the combustion chamber of a gun where, a fuel, such hydrogen, ethylene or kerosene, is burned with oxygen, and the heated and softened powders are expelled as a spray with the supersonic gases. Powders deposited using HVOF includes pure metals, metal alloys, cermets and certain ceramics and polymers. The reason why HVOF is the preferred thermal spray process for chrome replacement is because it produces low porosity (<1%), highly adherent (bond strength > 50 MPa) coatings which generally have an oxide content less than 1% even for reactive metals. As a flexible dry-coating technology it avoids high volume waste streams associated with electroplating and provides a choice of coating materials for each application.

CrC–NiCr and WC–Co systems constitute two main carbide materials used in thermal spraying processes in order to improve the wear resistance and decrease the friction coefficient between various sliding components in automotive applications. Furthermore, the CrC–NiCr system coatings are widely used in high temperature-wear resistance and corrosion-resistant applications in aerospace and powder engineering industries [1], [2], [5]. The CrC–NiCr coatings can be used in corrosive environments at service temperatures up to 800–900 °C [3]. Coatings of the WC–Co system generally have a higher hardness and wear resistance than CrC–NiCr coatings, however, the decarburisation of WC into W₂C, W₃C and even metallic W phase leads to the degradation of coating properties and limits the application of these coatings to temperatures below 450–530 °C [3]. The main shortcoming of CrC–NiCr coatings is a lower hardness than WC–Co system coatings [4].

The high-velocity oxy-fuel (HVOF) thermal spraying process has shown to be one of the best methods for depositing conventional Cr₃C₂–NiCr feedstock powders, because the hypersonic velocity of the flame shortens the time of interaction between the powder and the flame. These effects in conjunction with the relatively low temperature (as compared to plasma based techniques) result in less decomposition of the carbide particles during spraying [7]. In addition, the high kinetic energy acquired by the powder particles ensures a good cohesion in the coatings and allows producing carbide based coatings with a minimum porosity and decarburisation [6]. However, the effects of feedstock powder characteristics, type of HVOF spray system and spray parameters have been shown to affect the coating microstructure and consequently its wear resistance [8], [9].

The objective of this work is to conduct applied research and development to demonstrate that CrC–NiCr HVOF coatings are equivalent or superior in performance than chrome plating. The “problem solving” of this study is the replacement of hazardous hard chromium plating technology used today in the industry to coat different type of critical mechanical components (valves, pistons, piston rings, rods, hydraulic components) with an efficient and economic “clean” HVOF technology. The achievement of superior wear resistance from CrC–NiCr coatings would require an optimization of the powder preparation and an adaptation of the spray process. In the present investigation, novel CrC–NiCr coatings deposited with three different Cr₃C₂75–NiCr25 agglomerated feedstock powders with various powder size distributions have been studied. The properties of above mentioned HVOF coatings have been compared with the properties of conventional hard chromium plating.

Section snippets

Materials

The coatings studied in this work were Cr₃C₂ 75% + NiCr20 25% weight deposited on a steel substrate with a thickness of approximately 150 μm, using a high-velocity oxy-fuel system (HVOF). The used powders were prepared by WOKA. The WOKA 7204 spherical, agglomerated and sintered powder (75% chrome carbide – 25% nickel chrome) was used as standard powder. Three different 2075-NiCr powders were used as feedstock powders in the present investigation, namely (a) Standard (b) Fine-10 μm and (c) Fine-5 μm.

Morphology and microstructure of the powders

The morphological differences between the Cr₃C₂–NiCr powders before and after the attrition process are shown in Fig. 1. The standard powder had a predominantly spherical morphology (Fig. 1a), in the size range 10–30 μm. Mechanical milling of the standard spherical Cr₃C₂–NiCr powders led to the formation of two powder morphologies exhibiting a mixture of spherical and irregular agglomerates, as shown in Fig. 1b. This morphology is attributed to the continuous melting and fracturing of the powder

Conclusions

Three CrC–NiCr thermal spray powders have been deposited by HVOF spraying to form coatings. The mechanical and tribological properties of the coatings depend critically upon the characteristics of the powders in terms of agglomerate size and carbide size.

Although the Fine CrC–NiCr agglomerates, which show a higher decomposition during spraying, produce coatings with lower hardness, the wear behaviour of these coatings is up to 50% better than standard CrC–NiCr coating. In Fine coatings the

Acknowledgements

The authors would like to take this opportunity to thank the European Commission, which founded this work through the Competitive and sustainable Growth programme (1998–2002). Project: “The replacement of hard chromium coatings for mechanical components through High Pressure Nano structure HVOF coatings”, under Contract G5RD-CT-2000-00231.

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HVOF coatings as an alternative to hard chrome for pistons and valves

Abstract

Introduction

Section snippets

Materials

Morphology and microstructure of the powders

Conclusions

Acknowledgements

Wear

Wear

Nanostruct. Mater.

Thin Solid Films

Thin Solid Films

Surf. Coat. Technol.

Wear

Wear

Chromium carbide coatings produced with various HVOF spray systems

Thermal Spray Science and Technology

Abrasion, erosion and scuffing resistance of carbide and oxide ceramic thermal sprayed coatings for different applications

Wear