A State-of-the-art overview - Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools

Manufacturing Rev. 2014, 1, 24 Ó Z. Zhang and H. Dong, Published by EDP Sciences, 2015 DOI: 10.1051/mfreview/2015001 Available online at: http://mfr.edp-open.org OPEN REVIEW ARTICLE ACCESS A State-of-the-art overview Recent development in low friction and wear-resistant coatings and surfaces for high-temperature forming tools Zhenxue Zhang* and Hanshan Dong The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK Received 20 December 2014 / Accepted 13 January 2015 Abstract – Ever-increasing demand for severe work conditions, high-performance and long-life has been drivers for developing new low-friction and wear resistant coatings and surfaces for high temperature forming tools. This paper provides an overview on the recent development of coatings and modified surfaces to minimise friction and wear between contact surfaces in high-temperature working environments. This overview covers the major progresses in: (1) nanocomposite coatings based on transition metal nitrides and oxides, and the blending with solid lubricants such as silver and (2) multi-layered coatings with different combinations of hard and/or lubricating layers and duplex systems with solid lubricants on textured hard coatings and nanotubes. The challenges and future directions towards long-life and sustainable low-friction and wear-resistant coatings and surfaces for high-temperature forming tools are discussed. Key words: High temperature, Lubrication, Low friction coating, Nanocomposite, Forming tools 1. Introduction The ever increasing demand for quality, flexibility, productivity and environmental performance has introduced new machining and forming technologies, including near net shape production, high speed machining, hard turning, micromanufacturing, dry machining and environmentally conscious machining. The successful implementation of many of these processes demands high performance from the forming tools. The tools have to resist rough conditions, such as the cyclic contact with the rapid-heated materials at temperatures above 1000 °C, abrasive wear by hard particles like scale and oxides, in combination with adhesive wear by high pressure loading [1]. In some forming processes, the severe adhesion between the mould and the working materials becomes a major difficulty for demoulding without applying releasing agents or lubricants. Therefore the production of materials that exhibit low wear rate and low coefficient of friction (CoF) over a wider range of working conditions is a big challenge in modern industrial tribological systems. This technical challenge is relevant not only to metal forming but also to industrial processes including tooling, aerospace parts and assemblies like *e-mail: propulsion bearing and fasteners and moving assemblies for hypersonic aircraft, missiles and nuclear industries. In order to improve the performance of the tools, an ideal tool material should combine properties like high hardness, good toughness and chemical stability. Moreover, the material has to cope with thermal loading by being more refractory, or by generating less heat during machining (i.e. reducing both shear and frictional energies), and/or by taking away the generated heat rapidly. With this regard, monolithic tool materials are being replaced by coated tools to enhance their performance. The coatings form a barrier between the tool and the work piece, preventing the tool material from being exposed. This reduces the diffusion rate and lowers the chemically and thermally induced tool wear such as adhesion and oxidation. Coatings can also improve the tools behaviour by altering the coefficient of friction, and increasing the hot hardness, resulting in lower abrasion rate [2]. 2. Potential coatings and solid lubricants for high temperature applications Traditionally, thermo-chemical surface treatments, i.e. hardening, nitriding, carbonitriding, are used to improve the wear resistance of forming tools. Hard and corrosion-resistant This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 2 Z. Zhang and H. Dong: Manufacturing Rev. 2014, 1, 24 coatings like nitrides and carbides of titanium and chromium (TiN, TiCN, TiAlN and CrN) are frequently used to protect the tools and extend their lifetime. These coatings are suitable to avoid galling problems, to prevent forming tool wear and to lower tool maintenance costs. More recently, solid lubricating coatings such as DLC and metal-MoS2 composites are being used to reduce friction. Thin films and coatings based on hard and/or solid lubricant materials have been widely used on tribological components to reduce friction, wear and debris generation, and they can be classified into five categories: (1) carbon based materials e.g. graphite, DLCs and nanocrystalline diamond; (2) refractory materials e.g. titanium and chromium based nitrides and carbides, oxides (Al2O3) and cubic boron nitride (cBN), (3) transition metal dichalcogenide compounds (MoS2 and WS2); (4) polymer (e.g. PTFE) and (5) soft metals like silver, copper, lead, tin, indium and gold [3]. Tin, indium, lead and PTFE have very low melting temperatures which make them inherently unsuitable for high temperature use. On the other hand, silver, copper and gold perform well as solid lubricants at moderately temperature (300–500 °C). Molybdenum disulphide is the first choice of lubricant in vacuum or dry air environments, but it oxidises rapidly in moist air and acts as an abrasive at room temperature. Furthermore, it is not lubricious at temperatures beyond 350 °C. Compared with MoS2, WS2 exhibits higher thermal stability and provides a 100 °C increase in the maximum operating temperature. In a typical WS2 structure, a layer of tungsten atoms is sandwiched between two layers of hexagonally packed sulphur atoms. The bonding between W and neighbouring S atoms is covalent, while consecutive layers of S are held together by weak van der Waals forces, resulting in inter-lamellar mechanical weakness. Thus, under a shearing force, the basal planes slide over one another by intra-crystalline slip and material is transferred to the counterpart rubbing surface, imparting low CoF of 0.02–0.06 in either dry inert gas or in ultrahigh vacuum. Conversely, when sliding in humid air, dangling or unsaturated bonds on the edge of basal planes react with moisture and oxygen in the environment forming WO3, which has a higher CoF (>0.15) [4]. For high temperature applications (>500 °C), a few oxides (PbO and B2O3) or fluorides (CaF2 and BaF2) are generally used as solid lubricants [5]. However, the shear strength of these materials is relatively high at low temperature, and the incorporation of these phases in a composite generally leads to high coefficient of friction throughout the temp (...truncated)