Contents lists available at ScienceDirect Diamond & Related Materials journal homepage: www.elsevier.com/locate/diamond Load sensitive super-hardness of nanocrystalline diamond coatings Grazia Cicala a , Vittorio Magaletti b , Giuseppe Carbone c , Giorgio Saverio Senesi a, ⁎ a CNR-ISTP (Istituto per la Scienza e Tecnologia dei Plasmi), Via Amendola 122/D, 70126 Bari, Italy b SITAEL S.p.A., via S. Sabino 21, 70042 Mola di Bari, BA, Italy c DMMM-Politecnico di Bari, Viale Japigia, 182 70126 Bari, Italy ARTICLE INFO Keywords: Nanocrystalline diamond coating Nanoindentation Hardness Atomic force microscopy Raman spectroscopy ABSTRACT Synthetic diamond films have attracted great attention for their extreme properties and potential engineering applications as protective and wear-resistant coating for cutting tools. Nanocrystalline diamond (NCD) coatings were synthesized from CH 4 /H 2 /Ar (1/10/89%) microwave plasma at four deposition temperatures (T D ) ranging from 653 to 884 °C. The hardness (H) and Young's modulus (E) of NCD coatings measured at three different loads (10, 25 and 47 mN) depended on the nanoindentation load-level. The NCD coating produced at the lowest T D showed values of H = 121 ± 25 GPa and E = 1036 ± 163 GPa at the highest load. This result was attributed to the formation of elongated nanocrystallites at low deposition temperature. Further, the NCD coating obtained at lower deposition temperature exhibited an anomalous indentation size effect (ISE), i.e. a reverse ISE (RISE), which was ascribed to the heterogeneity of grain sizes along the [220] and [111] directions. Finally, a positive and negative (inverse) Hall-Petch behavior was observed for grain sizes along the [111] and [220] directions, respectively. 1. Introduction Modern high-tech materials and coatings for engineering applica- tions require very strict features such as hardness, resistance to wear and erosion and durability. In this context a renewed interest is growing in the scientific community for the production of synthetic micro- and nano-crystalline diamond (MCD and NCD) coatings [1–7] by chemical vapor deposition (CVD). In particular, NCD coatings feature a smoother surface and a lower friction coefficient than MCD ones [8]. The low surface roughness and the superior mechanical properties of NCD allow many potential applications including the use as protective and wear- resistant coatings for cutting tools, thus avoiding time consuming and costly post-polishing and the use of components in micro-electro- mechanical systems (MEMS). The quality of NCD films in terms of wear-resistance and protection efficiency is mostly based on their mechanical properties such as hardness (H) and Young's (elastic) modulus (E) that can be measured by commercial nanoindentation testing systems. Reliable H and E values are obtained when the following four requirements are fulfilled: 1) the indenter area function is well calibrated; 2) the indenter is sharp and not blunt; 3) a minimal thermal drift between loading and unloading cycles is achieved using an appropriate pause time (10 s); 4) the tested samples exhibit a low surface roughness. However, the mechanical properties measured by a nanoindentation tester with a pyramidal geometry exhibit a scale-dependent behavior at penetration depths lower than 1 μm and/or for small size of the hard- ness impression, which result in an indentation size effect (ISE), i.e. an increase of H and E values with decreasing the load ([9] and refs therein, [10]). Several materials, including crystalline materials such as metals and soft alkali halides, ceramics, semiconductor materials, amorphous materials and alloys, frequently show the ISE, whereas other materials show the reverse ISE (RISE), i.e. a decrease of H and E values with decreasing the load [11]. The cause of ISE and RISE is not yet clearly and fully understood, although the most obvious and sim- plest interpretation of these effects is based on the omnipresent friction at the interface between the indenter and the tested specimen [12]. However, the ISE may also be attributed to material features, such as cracking, non-dislocation mechanisms, phase transformation and pre- sence of a hard layer at the surface [13]. Differently, the RISE is usually attributed to the testing systems, such as vibrations, and to the small size of the indent footprint which is difficult to assess due to the in- herent limits of optical microscopy. The mechanical measurements of nanocrystalline materials strongly depend on their chemical composition, structure and morphology. In particular, hardening and softening have been observed with dimin- ishing the grain size, which are related, respectively, to the well-known https://doi.org/10.1016/j.diamond.2019.107653 Received 5 July 2019; Received in revised form 5 November 2019; Accepted 30 November 2019 ⁎ Corresponding author. E-mail address: giorgio.senesi@cnr.it (G.S. Senesi). Diamond & Related Materials 101 (2020) 107653 Available online 03 December 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved. T