Surface Microstructure of Mo(C)N Coatings Investigated by AFM

The high-resolution method of scanning electron microscopy (SEM) does not allow significant differentiation in the microstructure of the Mo(C)N coatings obtained with different volume of acetylene in the vacuum chamber. The main structural features of these coatings at the magnification of ×5000 are microdroplets, craters after chipping of microdrops, spherical particles of oxides (Fig. 1).

The fraction of microdroplet phase in Mo(C)N coatings amounts to several percent, and it does not manage to determine the differences according to this criterion. When assess the microstructure of coating surface according to SEM images with magnification of ×25000, it is only possible to make conclusion that protruding borders of cells are the most developed and have the smallest cells size about 600 nm for the coating with 7.1 at.% of carbon. In coating with 13.5 at.% C, the borders are poorly visible. The developed borders in this coating are very rare. In coating with 18.5 at.% C, the borders achieve the biggest size of about 1 µm (Fig. 2).

The use of AFM on the scanning field with size from 20 × 20 µm till 40 × 40 µm enables to assess only the depth of craters after chipping of microdrops (Fig. 1b and d). It is 50-120 nm. Like in SEM images, in AFM images the largest crater diameters of about 5 µm are in the coating obtained with 5 sccm acetylene flow. The smallest crater diameters of about 2 µm are in coating with 20 sccm C₂H₂ flow. The coatings obtained with 10 and 15 sccm acetylene flow have irregular distribution of craters (Fig. 1b and d).

The values of roughness Mo(C)N coatings obtained by AFM on areas 40 × 40 µm are in Table 2. It should be noted that roughness determined using AFM technique for Mo(C)N coating deposited without acetylene flow rate in the same technological conditions is similar. For Mo_0.68N_0.32 Ra was determined using contact recording instrument (Hommel Werke T8000) and amounts to about 0.04 µm (Ref 22).

But in the case of AFM, undetected features of the coating which is formed under C₂H₂ flow are only the problem of the right size scanning area selection. The use of the scanning area 5 × 5 µm enabled to register the difference in the microstructure of Mo(C)N coating formed at different acetylene flow rate in vacuum chamber (Fig. 3). That difference reflects the influence of the main deposition parameters and mainly the energy of bombardment on the coating formation. Two competing processes occur—deposition and resputtering of the coating. As an effect of resputtering are well-shaped cavities. The border of grain as a more harder material is resputtered slower than the softer body of grains. During the ion bombardment, the atoms from horizontal surfaces are resputtered easier than the atoms from vertical surfaces. Therefore the surface obtained by AFM as 3D images is interpreted as a multi-cell structure in which lowered bottom of the cells (core grain) and elevated vertical grain boundaries are visible.

The microstructure of MoN coating obtained without acetylene consists of “cells” characterized by relatively large size—about 2 µm (Fig. 3a), completely randomly arranged. In coating deposited at 5 sccm C₂H₂ flow, the cells with smaller size, about 700 nm (Fig. 3b), are formed. The increased flow rate of the acetylene in the vacuum chamber results in additional carbon atoms which give rise in the number of crystallization nuclei on the surface of the formed coating. The density of the crystallization centers grows, and the formation of ridges is faster than without acetylene. The growing ridges form ordered cells of smaller size than without carbon. Around one cell, there are usually about six other cells. As a result, grains in size of about 100-200 nm appear in the microstructure of the coatings deposited with 10, 15, 20 sccm of acetylene flow. The carbon content increase in the coating leads to more ordered microstructure. In coating deposited at 10 sccm C₂H₂ flow, the grains are located randomly (Fig. 3c), but in coating deposited at 15 sccm C₂H₂ flow the fragments of linear chain of grains can be seen, Fig. 3(d). In coating obtained with 20 sccm C₂H₂ flow, small grains with diameter of 100-200 nm are located into border of grains sized in a range 0.5-1.5 µm (Fig. 3e). In this coating, the grains are ordered into the honeycomb-like structure.

The usage of AFM regime “Torsion” (or Lateral Forces) allows to visualize different phases on the surface of Mo(C)N coatings. The comparison of AFM data in “Torsion” regime with XRD data allows to get a distribution map of different phases in Mo(C)N coating (Ref 20).

According to the phase diagram and XRD results, the coating without carbon is characterized by cubic Mo₂N phase. But in the range of angles 72°-82°, some diffraction lines exist, which are broadened, shifted and overlay themselves (Fig. 4).

Despite the fact that according to the diffraction pattern on the surface of coating there are not any other phases, in AFM images (Fig. 5a) in the structure of MoN coating three types of phases can be visible: gray phase (indicator 1, Fig. 5a), which is both the background for white phase in size of about 100-400 nm (indicator 2, Fig. 5a) and dispersed black phase in size of about 50 nm (indicator 3, Fig. 5a). It can be supposed that the black phase is MoN. The white phase can be not quite stoichiometric phase or can be formed by some individual parts in the coating with the main cubic Mo₂N phase.

Based on the results (Ref 19) in which it was found that during nitriding of molybdenum in terms of saturation with nitrogen to temperature of 940 °C two layers: the outer layer of MoN and inner of Mo₂N were formed on the surface, we assume that fine black grains (phase) can be attributed to MoN. The same effect occurs in case of tungsten (Ref 23).

The existence of different phases on the coatings surface can be confirmed by findings of Yuan et al. (Ref 15) who suggest that carbon atoms substitute nitrogen atoms in the nano-sized crystalline Mo₂N to form Mo₂N(C) solid solution phase.

In fact the C₂H₂ flow of 5 sccm during deposition does not change the phase composition of the coating. It leads to the increase in formation of all nitride phases. Based on the intensity of the diffraction lines such as (111) expected at 33.38° for Cu-Kα radiation, (200) at 43.45° or (220) at 63.11° according to ICDD 25-1366 it can be stated that this coating consists mainly of the cubic Mo₂N phase. In contrast, the next diffraction line (311) positioned at 75.73° is more interesting. Its intensity increases and slightly broadens. It may be caused by the formation of MoC and/or MoN phase, but the amount of these compounds could be insufficient to be register by XRD. Such an increase in formation of nitride phases with the addition of small amount (5 sccm C₂H₂ flow) of carbon is reflected in the structure in form of grain size decrease, what is apparent in AFM image. The increase in carbon concentration in Mo(C)N coatings leads both to the appearance of larger crystallization centers quantity on the surface and to the decrease in grains size. In the microstructure of the surface, the grain size decrease is visualized in form of cells size decrease from 3 to 5 µm for molybdenum nitride coating without carbon (Fig. 3a) to 1-2 µm for molybdenum nitride coating with carbon amount of 7.1 at.% (C₂H₂ flow rate of 5 sccm), Fig. 3(b). There are also more developed ridges of grains in this coating. The quantity of ridges increases, and it leads to surface roughness rise from 29.3 to 46.5 nm on the area of 40 × 40 µm² (Table 1). In AFM “Torsion” regime, the quantity of phases, which are contrasted with the main phase, increases. Moreover, their size also decreases and the main phase is Mo₂N. In AFM image, the phase has gray contrast and consists of nanosized subgrains with diameter of 20-50 nm (indicator 1, Fig. 5b). As it was mentioned above, the diffraction lines are broadened and shifted to lower angles from their positions. It can be associated with the nanosized structure of coating. They are different in forms and color types of particles from the others phases in the background of main Mo₂N phase as dispersed dark particles with diameter of 20-50 µm so as lamellar bright particles with diameter 300-500 nm (indicator 2, Fig. 5b). They confirm the possibility of the existence of MoN and MoC phases (Fig. 4b). For the lamellar bright particles determined as MoC phase, the increase in size is more significant.

The addition into vacuum chamber 10 sccm C₂H₂ flow leads to the formation of significant quantity of MoC phase, which reaches the quantity of nitrogen phases. The deconvolution of complex and broadened diffraction lines in 2Θ angle range 70°-87° revealed the existence of MoN and MoC phases except for present Mo₂N phase. The quantity of the phases is approximately equal.

The ridges are visible (Fig. 3) for Mo_0.68N_0.32 and Mo_0.64C_0.07N_0.29 coatings which do not show carbon phases in XRD diffraction pattern, Fig. 5. The increase in carbon in Mo(C)N coatings decreases the grain size to about 100-200 nm (Fig. 3c). Such surface microstructure means that acetylene (source of carbon) introduced into the vacuum chamber has not only distributed on the surface in the form of crystallization nuclei, but its quantity is sufficient for stoichiometric MoC phase. In “Torsion” regime, there are dark particles dispersed, probably MoN (indicator 2, Fig. 5c) and white lamellar inclusions presumptive MoC phase (indicator 3, Fig. 5c). The first phase particles dimension is about 20-50 nm, and the white phase is about 400-600 nm. Based on observation of the microstructure in the lateral force regime, it can be noted that the number of phases in both colors increased. They are uniformly distributed on the surface.

The increase in the acetylene flow rate in the vacuum chamber to 15 sccm results in achieving the maximum intensity of MoC phase diffraction lines in the diffraction pattern. The further rise of the acetylene flow rate to 20 sccm does not change intensity in MoC diffraction lines. Probably, the formation of MoC phase could be finished. A transformation of the main Mo₂N phase to mixture of Mo₂N, MoN and MoC phases takes place. The structure type is changing drastically (Fig. 5c). The dispersed phase appears fast in the coating with a uniform grain size of 100-200 nm (indicator 1, Fig. 5c). Bright and dark particles (indicator 2, Fig. 5c) are uniformly distributed on the surface. We suppose that lamellar bright particles of MoC phase compared with one shown in Fig. 5(b) have more developed surface. Their size decreased significantly to 100-400 nm (indicator 3, Fig. 5c).

In the Mo_0.53C_0.19N_0.28 coating formed in vacuum chamber in acetylene flow rate of 20 sccm, the surface microstructure type, that is dispersed, grains do not change (Fig. 5e) and are the same as for coating formed in 15 sccm of acetylene. The difference is in grain size—in this coating they were decreased. The size of dark, probably MoN, dispersed phase is 50-100 nm (indicator 1, Fig. 5e). The other particles MoC (indicator 2, Fig. 5e) and Mo₂N (indicator 3, Fig. 5e) have similar size—100-200 nm. The main structure of dispersed particles is ordered. The grains are arranged into a honeycomb structure with a cell size of about 1 µm. The size of phases observed in AFM measurements in all coatings is gathered in Table 3.

Due to refinement of grains and for proper assessment of grain dimensions, the AFM measurements on the square 10 × 10 µm ought to be taken. The roughness resulted from these tests reflects the features of coating formation correctly. The features of microstructure of Mo(C)N coating like microdrops significantly affect the value of roughness on square 20 × 20 µm or more. Its surface distribution has a random nature. The smaller size of square causes incorrect cells structure. So the coating with 7.1 at.% C shows 19 nm roughness, while the common MoN coating is only 10 nm. This confirms an idea that at acetylene flow rate of 5 sccm in vacuum chamber, MoC phase does not form. Instead of that, the grain refinement of nitriding phases occurs and as an effect the more shaped grain boundaries are formed and the additional ridges appear on the surface. Therefore, the value of roughness decreases to 11.2 nm for coatings deposited at C₂H₂ flow rate of 10 sccm and to 9.4 nm at 15 sccm. The MoC phase formed as the rounded nanosized grains smoothes the coating surface. The further increase in roughness to 15.2 nm is connected with the appearance of chains and ridges from grains.

Hard coating shows usually high wear resistance, and as consequence the depth of wear tracks was several nanometers. The relief of the surface and craters were more apparent than the depth of obtained wear tracks. The results of wear tests on Mo₂N (acetylene flow rate—0 sccm) and Mo(C)N deposited using acetylene flow rate of 10 sccm are presented in Fig. 6. Below images of worn surface of the coatings the profiles of wear tracks are added.

The depth of wear (volume of material removed from the surface) was 56 nm (9.0 × 10⁻¹⁹ m³) for Mo₂N coating and 16 nm (2.6 × 10⁻¹⁹ m³) for Mo(C)N coating deposited using 10 sccm of acetylene. Specific wear rate calculated according to Archard’s formula is 14.6 × 10⁻¹⁴ m³/N m and 4.2 × 10⁻¹⁴ m³/N m, respectively. It means that MoCN coatings can present better wear resistance compared to Mo₂N coating.