NEW GENERATION OF HIGH PERFORMANCE COATINGS FOR MIRROR FINISH SURFACES

INTRODUCTION

Nowadays coatings such as Chrome nitride, Titanium nitride, and Zirconium nitride are fully accepted with good reliability for many mechanical applications just like they are for decorative applications.

However, even if their composition is identical, it remains difficult to combine the properties of the functional coatings which were developed for mechanical applications, with the surface properties of the decorative coatings existing on the market.

The objective of the studies led by DMX Research on this theme was to work on the implementation of a hybrid coating process allowing the development of high performance technical films offering levels of roughness compatible with mirror finish or optical mirror finish surfaces.

APPROACH

Various parametric studies were led with the aim of obtaining coating solutions with the following characteristics:

  1. High hardness (> 20GPa)
  2. Limited internal stress
  3. High level of adhesion on various substrates
  4. Smooth coatings with very low surface roughness obtained without post-treatment such as brushing, blasting, drag finishing or polishing.

The work plan followed for the development of such coating solutions was based on a global approach of the problem. The cleaning and the preparation of the tooling as well as the various stages of chemical and physical cleanings/etchings were indeed studied and adapted to satisfy the multiple technical constraints inherent to these extremely sensitive polished surfaces.

Then we focused on the control and the full understanding of the growth of the coatings by utilizing all the options offered by the implemented hybrid technology.

The coatings obtained were applied on silicon and high speed steel substrates compatible with most of the necessary characterizations but also on typical steel substrates often used in the industry like X13T6W and Z38CDV5.

CHARACTERISTICS OF THE COATINGS DEVELOPED

The production of coatings offering an extremely low level of roughness can only be achieved by controlling their growth and their microstructure.

The fig. 1 presents examples of microstructures obtained with various combinations of parameters. These studies have demonstrated the versatility of the developed hybrid processes with the possibility to produce coatings with an open columnar structure (configuration A) as well as coatings presenting much denser microstructures (configurations B and C).

Fig 1: Images (MEB FEG) of ZrN coating surfaces produced with various configurations of coating process parameters (Si substrate, coating thickness between 1,7µm and 2,3µm)

Fig 1: Images (MEB FEG) of ZrN coating surfaces produced with various configurations of coating process parameters (Si substrate, coating thickness between 1,7µm and 2,3µm)

The optimization of the combined technologies of the hybrid process and thus the microstructure of the coatings as well as the control of the quality of the surface preparation allowed minimizing the risks of growth defects and producing coatings with very promising roughness levels.

The graphs of fig. 2 & 3 present respectively the differences of surface roughness Sa and Sz measured by optic interferometry on mirror finish steel substrates before and after coating, for the three compositions of coatings studied (TiN, CrN and ZrN).

These measurements highlight very low surface changes for all three coatings. We indeed noticed, that for coating thicknesses between 1,5 and 2,5µm, the Sa values are very lightly impacted and differences on the Sz values remain extremely low.

Fig 2: Chart representing the surface roughness Sa (optic interferometry 50x, average values obtained on 10 measurements) from steel substrate (X13T6 mirror finish) before and after coating for thicknesses between 1,5 and 2,5µm

Fig 2: Chart representing the surface roughness Sa (optic interferometry 50x, average values obtained on 10 measurements) from steel substrate (X13T6 mirror finish) before and after coating for thicknesses between 1,5 and 2,5µm

Fig 3: Chart representing the surface roughness Sz (optic interferometry 50x, average values obtained on 10 measurements) from steel substrate (X13T6 mirror polish) before and after coating for thicknesses between 1,5 and 2,5µm

Fig 3: Chart representing the surface roughness Sz (optic interferometry 50x, average values obtained on 10 measurements) from steel substrate (X13T6 mirror polish) before and after coating for thicknesses between 1,5 and 2,5µm

Images of fig. 4 show the 3D profiles of the coated surfaces measured by optic interferometry on the same steel substrates.

The mechanical characterizations realized by nano-indentation (see fig. 1) and by scratch test (see fig. 5) on these same coatings confirmed that the mechanical properties of these layers are equivalent to those of the functional coatings of the same composition used today for mechanical applications.

The characterization example obtained with the scratch test in the case of the CrN coating shown on fig. 5, illustrates the high level of adhesion of the coating (critical load around 60N).

Table 1: values of hardness and Young modulus measured by nano-indentation (High Speed Steel substrates; average values on 30 indents; Berkovitch diamond tip; Oliver and Pharr method)

Table 1: values of hardness and Young modulus measured by nano-indentation (High Speed Steel substrates; average values on 30 indents; Berkovitch diamond tip; Oliver and Pharr method)

Fig 4: 3D profile (optical interferometry 50x) of mirror finish X13 T6W steel coated with TiN, CrN and ZrN

Fig 4: 3D profile (optical interferometry 50x) of mirror finish X13 T6W steel coated with TiN, CrN and ZrN

Fig 5: Scratch test (optical microscope) realized on High Speed Steel substrate test piece (65HRc) with mirror finish coated with a thin film of CrN with 2µm thickness (length of scratch test is 3mm, load increment of 158N.min)

Fig 5: Scratch test (optical microscope) realized on High Speed Steel substrate test piece (65HRc) with mirror finish coated with a thin film of CrN with 2µm thickness (length of scratch test is 3mm, load increment of 158N.min)

CONCLUSIONS

The development of our hybrid coating process allowed us to obtain coatings with extremely good surface properties (low roughness) and very good adhesion while maintaining optimum mechanical characteristics.

These coatings processes can be applied at low temperature (190°C) and in a standard clean environment (no obligation to work in clean-rooms). This is why they are optimum solutions for various industrial applications such as the protection of plastic injection molds with high quality surface finish (mirror finish or optical mirror finish).