Diffusion-modified boride interlayers for chemical vapour deposition of low-residual-stress diamond films on steel substrates

Abstract

The feasibility of using a boriding pretreatment for the chemical vapour deposition (CVD) of adherent, low-residual-stress diamond films on ferritic tool and AISI type 316 austenitic stainless steels was investigated. The steel samples were borided by means of a pack cementation process at a temperature of 950 °C using an interrupted thermal cycling process. Boriding of the alloy steels results in a very high surface hardness of approximately 3780 VHN due to the precipitation of alloy borides such as chromium boride in the predominantly FeB and/or Fe₂B case. The boriding conditions, and hence the microstructural state of the as-borided steels, was found to have a strong influence on the diamond film characteristics, particularly on the adherence. Detailed characterisation of the as-borided steels, as well as the deposited diamond films and interlayer modification during the CVD process, is discussed based on scanning electron microscopy, X-ray diffraction and micro-Raman spectroscopy investigations. Under optimised conditions, adherent and continuous diamond films of good quality have been obtained on both the ferritic tool and austenitic stainless steels. In the case of borided surface structures without the presence of a FeB phase, diffusion-modified gradient microstructures were found to accommodate efficiently the high thermal stress expected between the steel substrate and the diamond film, resulting in low-residual-stress films.

Introduction

At present a number of different chemical vapour deposition (CVD) techniques are available to deposit polycrystalline diamond coatings on a wide range of substrate materials. The objective of the present work was to deposit CVD diamond coatings on steels, as these can provide cost-effective substitutes for cemented carbides and other hard tools for industrial applications. However, on direct deposition on blank steel, the enhanced tendency for graphitic nucleation on iron-based surfaces results in the formation of a thick graphite layer prior to diamond nucleation [1]. This leads to poor adhesion of the diamond film, while the high carbon diffusivity in iron also results in an increased incubation time for diamond film formation. The use of suitable interlayer coatings or surface modifications to aid diamond nucleation and to act as diffusion barriers to prevent degradation of the diamond film formed, which could be caused by the diffusion of iron from the bulk, is considered essential [2]. Another major constraint in obtaining continuous and adherent diamond films on steel follows from the large difference in their thermal expansion coefficients. This, in combination with the high elastic modulus of diamond, leads to very high thermal stresses during cooling down, resulting in an increased tendency for delamination and/or cracking of the diamond film. Particularly since the thermal expansion coefficients of austenitic stainless steels are nearly twice those of ferritic tool steels, deposition of uniform diamond films on austenitic steels has been found to be difficult [3], [4]. Hence, for diamond deposition on steel substrates the interlayer chosen should have a desirable microstructure and the ability to accommodate the high thermal stresses induced. It is important to choose an interlayer that results in increased adhesion and that provides the ability to nucleate diamond films at lower deposition temperatures as well, as this contributes to a further reduction in thermal stress.

Overlay coatings of the order of several μm, mainly produced by PVD deposition techniques, have been extensively explored as interlayer systems for diamond deposition on steel substrates. Some examples include the use of Mo [5], Ni [6], TiN [7] and CrN [8], [9] interlayers. However, the use of such overcoat interlayers results in an abrupt change in the elastic, thermal and mechanical properties at the interface. A sudden transition in the residual stress across a specific interface can lead to enhanced propensity for delamination during cooling, or failure during service exposure, particularly on austenitic steels. For example, it has been observed that the use of a PVD CrN coating can result in a continuous diamond film on tool steel substrates, but not on austenitic AISI 316 type stainless steels [8], which have higher thermal expansion coefficients. Even diamond films deposited on ferritic tool steel substrates coated with various kinds of interlayers still encounter high thermal stresses. For example, the use of a tungsten interlayer has been found to be efficient in depositing diamond films over ferritic steel substrates, although this results in very high residual stress of the order of approximately 7 GPa [10].

Although PVD coated ceramic interlayers act as good diffusion barriers [8], [11], the use of diffusion-modified interlayers in general has several advantages. Because of the gradual change in concentration of the in-diffused element and that of the mechanical properties of the modified substrate material, better bonding between the interlayer and the bulk substrate is obtained. The use of an interlayer system with a thermal expansion coefficient and elastic modulus in between that of steel and diamond is a key to obtaining continuous diamond films of high quality with good adhesion on steel. A composite surface structure integrated smoothly with the bulk and having a continuous gradient in properties, such as the elasticity, hardness and thermal expansion of the material, can result in better accommodation of the thermal stress and in improved adhesion of the diamond film on the steel substrate. This kind of microstructure can be obtained by diffusion-based thermo-chemical surface modification treatments, such as nitriding and boriding.

Conflicting results on the effect of (steel) substrate nitriding on diamond film growth have been reported. Schäfer et al. have reported delamination of diamond films deposited on nitrided high-speed ferritic steels [12]. However, Shang et al. [13] demonstrated the possibility of obtaining continuous diamond films on nitrided martensitic alloy steel. A diffusion-modified interlayer system based on gas-nitrided chromium on carbon chrome ferritic alloy steel was proven to be efficient by Glozman et al. [14]. However, as the chromium layer is deposited by electroplating, there is still an abrupt change in properties at the chromium/steel interface. They also observed very high residual stresses acting on the diamond films deposited, which is disadvantageous for engineering applications. Schwarz et al. reported on the application of high-temperature diffusion chromising as a successful method for CVD diamond coating of steel [15]. The chromium carbide intermediate layers, formed by a diffusion process, act as reasonably good diffusion barriers during the diamond deposition process, although graphite formation in certain areas is still observed. The diamond coatings also show very good adhesion, as assessed by means of scratch testing.

From the discussion above it is clear that some initial success has been achieved in obtaining continuous diamond films, but this is primarily in the case of ferritic steel substrates so far. Very few reports of uniform diamond coatings on austenitic steels have been presented [3], [4], [16]. Chen et al. [16] reported continuous adherent diamond films on AISI 304 austenitic steel substrates using a 100-nm-thick unspecified intermediate layer in combination with a low-temperature deposition process.

The present investigation aimed to study the feasibility of using a diffusion-modified borided steel surface for diamond film growth on both ferritic and austenitic steels. In general, the nitriding, carburising and boriding processes have been known for a very long time and are used as surface hardening pretreatments. Up until now, only the nitriding and carburising processes have been employed for diamond deposition onto steel [17]. It is known that boriding results in enhanced hardness and in increased wear, fatigue and corrosion resistance as compared to nitriding and carburising. For example, the Vickers hardness of carburised mild steel surfaces is reported to be approximately 1000 VHN, whereas the surfaces of boronised steels show values up to approximately 2000 VHN [18]. Hence, diamond coatings on borided steels might result in improved tribological properties as compared to those on nitrided or carburised steels. It is also known that iron borides are strongly resistant to carburisation, and therefore the formation of thick carbide intermediate layers during the diamond CVD process might be avoided by using borided steels. This work describes hot-filament-assisted CVD diamond deposition (HFCVD) on borided AISI type 316 stainless steel and tool steel substrates. The main emphasis of this paper is on the effect of boriding conditions on the adhesive properties and residual stresses of the diamond films grown.