NEW CARBON BASED COATINGS EVEN MORE FLEXIBLE

INTRODUCTION

Diamond Like Carbon (DLC) coatings remain even today one of the hottest topics in the industry and the world of academic research.

Industrial companies already adopted some of the DLC versions many years ago. Their performances, in particular for large batches for the auto sector, no longer need to be demonstrated.

However, difficulties to industrialize these coatings remain when moving away from the basic applications targeted by this type of layers.

Coating successfully different parts with various and complex geometries within the same batch is a current difficulty for coating companies. Coatings must have good adhesion and performance on various materials with a different surface finish.

DMX Research’s recent work has been driven by the aim of developing DLC coating processes with a high level of flexibility and reliability, for industrial applications.

NEW DEVELOPMENTS

1. A sensible compromise between surface roughness and adhesion

The weakness of most DLC coatings is the difficulty to obtain an excellent level of adhesion on various types of substrates, with simple to very complex geometries.

Specific coating processes developed and optimized by DMX allowed reaching excellent properties, in particular in terms of roughness and adhesion as shown respectively in figures 1, 2 and 3.

Figure 1: Image of the surface (MEB FEG x500) of the latest developed process (silicon substrate, thickness 1,5 µm)

Figure 1: Image of the surface (MEB FEG x500) of the latest developed process (silicon substrate, thickness 1,5 µm).

Figure 2: Photo (optical microscope 50 x) of Mercedes adhesion test (Rockwell C indentation at 150 kg) realized on the latest developed coating (thickness 1,5 µm) applied on a High Speed Steel substrate (65 HRc)

Figure 2: Photo (optical microscope 50 x) of Mercedes adhesion test (Rockwell C indentation at 150 kg) realized on the latest developed coating (thickness 1,5 µm) applied on a High Speed Steel substrate (65 HRc).

Figure 3 : Photo (optical microscope) of scratch test realized on a High Speed Steel substrate (65 HRc) coated with a 2 µm film (length of the scratch 3 mm ; load ramp 158 N.min)

Figure 3 : Photo (optical microscope) of scratch test realized on a High Speed Steel substrate (65 HRc) coated with a 2 µm film (length of the scratch 3 mm ; load ramp 158 N.min).

2. A significant decrease in growth failures

The optimization work also allowed to decrease significantly the growth failures (typically ” pinholes “) and thus to increase the capacity of these coatings to act like an effective barrier against certain corrosion phenomena.

The cross section of the coating film shown in figure 4, highlights this property.

Figure 4: Photo MEB FEG (50kX) of coating film cross section (Silicon substrate)

Figure 4: Photo MEB FEG (50kX) of coating film cross section (Silicon substrate).

3. Variability of the process under control

Finally, the particularity of the developed coating process is its ability to ensure that properties of the coating film are kept homogenous in regards to the number and geometry of the parts to be coated.

Two coating films realized with different conditions and different load configurations have been produced and characterized. The results of the main properties measured on test pieces are shown on table 1.

Table 1 : Properties of  coating films deposited in regards to the carousel’s configurations (permanent contact / friction test using pin on disk test  with Al2O3 ball/ PHertz = 1,15 GPa/ moving speed= 20 cm.s-1)

Table 1 : Properties of coating films deposited in regards to the carousel’s configurations (permanent contact / friction test using pin on disk test with Al2O3 ball/ PHertz = 1,15 GPa/ moving speed= 20 cm.s-1)

These results show a very light decrease in hardness and Young module in the case of the configuration 2, but both remain in a fully acceptable range (taking into account the limits of precision of the measurements).

These two coatings were also analyzed in spectrometry RAMAN. Spectral analyses obtained and main extracted data from their deconvolution are respectively presented on the graph of figure 5 and table 2.

Figure 5 : RAMAN spectra obtained for coatings produced with two different carousel configuration (Laser Ar 514nm / Power 7,5mW filtered at 25% / Average Result over 10 spectra).

Figure 5 : RAMAN spectra obtained for coatings produced with two different carousel configuration (Laser Ar 514nm / Power 7,5mW filtered at 25% / Average Result over 10 spectra).

Table 2: Values of the two RAMAN parameters (position of the band G and ratio of intensities of D and G peaks) for the two coatings.

Table 2: Values of the two RAMAN parameters (position of the band G and ratio of intensities of D and G peaks) for the two coatings.

These data confirm the similarity of microstructure of both coating films. The slight difference observed in the position of the band G (cf. table 2), highlighted by a small shift of the position towards the higher wave number for the carousel with higher load density, is consistent with the decrease of hardness noticed previously because it could be explained by a more important hybridization sp2 of the carbon.

CONCLUSIONS

The coating process created and developed by DMX results in an innovative and successful DLC coating suitable for numerous applications related to wear resistance, friction and corrosion protection.

The optimizations of this low-temperature coating process increased the versatility and reliability of the coating, thus opening new perspectives for a large number of applications with complex geometries and various materials with a different surface finish.