Very thin hard films of amorphous carbon (or diamond-like carbon (DLC)) are now widely used as protective overcoats for magnetic hard disks. This has stimulated a lot of work on adhesion assessment, as well as friction and wear properties of such materials.
This article features the results of nanoscratches performed on a typical hard disk surface, made up of different multilayers: these films consist of a 13 nm carbon overcoat on the top, with some magnetic, metallic and other films having a total thickness of about 120 nm. The work was done using the Nano-Scratch Tester from Anton Paar.
Taking into account such thicknesses, a spherical diamond was chosen with a small tip radius (2 μm) in order to generate a maximum stress field near the sample surface. In order to study the friction fatigue of the sample, a multipass constant load cycle was used. By successively increasing the number of cycles, N, under a low load (2 mN), fatigue failure assessment was possible. Scratches were made with N varying from 1 to 70. Damage was seen to initiate when N = 50 and rapidly increased until catastrophic damage occurred by blistering. To verify the adhesive character of the coating damage, the blistered damage area can be removed by making a scratch in a transversal direction to that of the original scratch. This can also provide valuable information about multilayer plastic deformation below the DLC overcoat.
Figure 1. Result of transversal scratches on a typical damage area having undergone 70 cycles with an applied load of 2 mN and spherical indenter of radius 2 μm. The optical micrograph (a) clearly shows the resulting decohesion and the SFM image (b) confirms the depth at which failure has occurred.
Figure 2. SFM cross-sectional profile through the image shown in Fig.1 (b). The depth at which coating failure occurs can be accurately measured as being 100 nm below the surface. This corresponds to the interface between the substrate and first coating.
Transversal scratches were performed with the Y-axis translation table of the NST, the same tip and an applied load of 1 mN. The result is shown in Fig. 1, where coating decohesion and chip evacuation can be observed. SFM analysis with transversal profiles allows an accurate study of the damage process; the SFM cross-sectional profile of the delaminated area (Fig. 2) confirms that the adhesion failure is situated at the interface between substrate and first coating, because of the known multilayer thickness.
For a more accurate study of adhesion strength of the DLC film, a stylus with a smaller tip radius (0.5 μm or less) must be used. Experimental studies with different tip radii are currently being investigated. The NST is particularly suited to ultrathin coating characterisation of this kind, owing to its wide range of contact conditions and available loads.
Under well-defined conditions, plastic deformation can be limited to the coating(s) with only minor effects, if any, on the substrate. This is especially important for the case of ductile or brittle coating/substrate composites.