Growth and characterization of integrated nano- and microcrystalline dual layer composite diamond coatings on WC–Co substrates Ravikumar Dumpala a, b , Maneesh Chandran b, c , N. Kumar d , S. Dash d , B. Ramamoorthy a , M.S. Ramachandra Rao b, c, ⁎ a Manufacturing Engineering Section, Department of Mechanical Engineering, Indian Institute of Technology Madras, Chennai 600036, India b Nano Functional Materials Technology Centre, Indian Institute of Technology Madras, Chennai 600036, India c Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India d Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, TN, India abstract article info Article history: Received 12 October 2012 Accepted 28 November 2012 Keywords: Hot filament CVD Composite coating Diamond film Nanocrystalline Microcrystalline In this work, integrated composite diamond (ICD) coatings have been achieved with top layer nanocrystallin- ity, low friction coefficient and enhanced integrity. ICD coatings were deposited on chemically treated tung- sten carbide (WC–Co) substrates using hot filament chemical vapour deposition technique. Nanocrystalline diamond (NCD) layer was deposited over microcrystalline diamond (MCD) layer with a coating architecture of NCD/transition layer/MCD/WC–Co. Graded transition layer thickness of ~1 μm was realized by controlling the process parameters such as methane concentration and chamber pressure in order to integrate the MCD and NCD layers. Integrity of the coatings was examined by the cross-sectional studies. Structural and micro- structural characteristics of ICD coatings were compared with those of MCD coatings. The measured average nanohardness of ICD coating was ~ 96 GPa. A low and stable friction coefficient of ~ 0.06 was observed for ICD coatings against silicon nitride (Si 3 N 4 ). ICD coatings were anticipated to exploit the advantages of both NCD and MCD coatings and these coatings can be promising candidates for various mechanical applications. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction CVD diamond has drawn great attention of the research community from the beginning of its invention through decades of development because of its unique mechanical, thermal, electrical and optical proper- ties with promising applications [1]. Years of research has led to the successful application of CVD diamond coatings on cemented tungsten carbide (WC–Co) materials by optimizing the cobalt content, tungsten carbide grain size and different surface treatment methods [2–6]. Conventional CVD diamond coatings with columnar growth lead to faceted grains with high surface roughness, which limits the direct prac- tical applications [7]. The diamond layer morphology and the growth process are controlled by the gas phase activation and thus by the depo- sition parameters. Methane concentration and chamber pressure typical- ly control the grain size of the diamond which spans the range from few micrometres to nanometres [8,9]. Smooth nanocrystalline diamond (NCD) surfaces are developed by reducing the grain size to the order of nanometres (10 to 100 nm), which are suitable for practical applications, but intrinsic stresses within a layer increase with a decreasing grain size [10]. Mostly graphitic and hydrogen contents at the grain boundaries are responsible for the generation of intrinsic compressive stresses, which contribute to major part of total residual stresses along with thermal stresses [11]. CVD diamond coatings with thicknesses as high as 25 μm are being used for machining applications [12]. Typically NCD coatings show nanocrystallinity with the coating thicknesses below 3 μm. The average grain size and surface roughness of the NCD coatings increase along with the increasing thickness [13,14]. But uniformity and control over the grain size are the typical requirements for many industrial applica- tions. Well crystallized MCD films show good adhesion to ceramic substrates, whereas NCD films show less effective adhesion due to the higher degree of sp 2 content. The lower the graphite content at the coat- ing substrate interface, the higher the bonding strength is [15,16]. Also NCD coatings have larger number of grain boundaries that contain sub- stantial amount of graphitic and hydrogen impurities which diminish the crystalline quality of the films and thereby affecting the mechanical properties [17]. Multilayer system is one of the reliable approaches to exploit the ad- vantages of both NCD and MCD layers. Several studies on multilayer composite diamond coating systems consisting of alternate layers of MCD and NCD are reported with low friction, improved surface rough- ness, good adhesion and mechanical properties [18–25]. However, without transition layer sharp interfaces of these diamond layers are more susceptible to separation during practical applications due to the difference in residual stresses among the layers and higher degree of graphitic content at the interface of NCD layer. Interfacial impurity Int. Journal of Refractory Metals and Hard Materials 37 (2013) 127–133 ⁎ Corresponding author. Tel.: +91 44 2257 4872; fax: +91 44 2257 4852. E-mail address: msrrao@iitm.ac.in (M.S.R. Rao). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijrmhm.2012.11.007 Contents lists available at SciVerse ScienceDirect Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM