Technische Akademie Esslingen, Proceedings on CD-ROM, 15-F8

15th International Colloquium Tribology, Automotive and Industrial Lubrication, 2006 January 17-19 in Stuttgart/Ostfildern Germany, Technische Akademie Esslingen, Proceedings on CD-ROM, 15-F8

Experimental and Numerical Analysis of the Degradation of Polycrystalline Diamond A. ELEÖD Budapest University of Technology and Economics, Budapest, Hungary [email protected] M. SCHMITT Institut de Chimie des Surfaces et Interfaces CNRS UPR 9069, Mulhouse Cedex, France SUMMARY Experimental and numerical investigations were performed to analyse the degradation of diamond coatings deposited by the combustion flame method on a tungsten carbide substrate. The crystal orientation of the polycrystalline diamond coating was uniformly {100}, this orientation ensures the best coating surface quality and does not require any polishing after deposition. Friction tests were performed using on one hand a pin-on-disc apparatus with diamond coated tungsten carbide pins and uncoated tungsten carbide disc as counter face, on the other hand, under conditions of milling using a magnesium alloy work piece. The worn diamond coating was analysed by SEM and Raman Spectroscopy. Parallel with the friction tests, to estimate the stress state as well as the contact temperature of the coating during sliding, numerical simulation was also conducted, using a nonlinear finite element model. 1 Introduction The most promising coating for hard metal cutting tools is diamond in any of a variety of forms: polycrystalline or nanocrystalline, single layer or multilayer coating, deposited with a polished surface or with an "as deposited" surface [1]. However, it is true for all types of diamond coatings that it performs excellently during cutting only as long as it remains intact. Thus it is essential to know the limiting values defining its performance, as well as the forms and causes of damage. The current work summarizes the results of many years of collaboration and deals with deposition technology, the development of the analysis and quantification methods, as well as the numerical analysis and control of the mechanical resistance of coatings. 2 Experimental Investigations were performed using a pin-on-disc apparatus and a commercial tungsten carbide hard metal cutting tool with polycrystalline diamond coating for machining tests. 2.1 Friction tests on pin-on-disc apparatus Friction tests were conducted on a CSM pin-on-disc tribometer allowing heating of the disc. 2.1.1 Counterfaces

Both hemispherical pins (8 mm diameter) and discs (57 mm diameter) were produced from tungsten carbide

(hardness = 72 HCR), chemical analysis: 88% WC, 10% Co and 2% TiC/TaC. The pins only were subsequently etched to reduce the surface Co content. This material was chosen for its high hardness and excellent surface state which limits material transfer so that the tribological behaviour of the diamond coatings was not masked by excessive of transfer. 2.1.2 Diamond coatings Earlier studies [2] have shown that from a tribological point of view it is worth utile to investigate {111} and {100} oriented coatings. We will see further that the polycrystalline diamond coatings used industrially are typically composed of a mixture of these orientations. However, for the present work we have considered only {100} oriented coatings. The diamond coatings used in this work were obtained with the combustion flame process, a method based on the dissociation of carbon from acetylene during its combustion in oxygen [3]. Unlike diamonds synthesized by CVD, the introduction of these two gases leads to the formation of dangling bonds which are mainly saturated by oxygen (instead of hydrogen). The substrate was a tungsten carbide cylinder with a hemispherical extremity. The diamond crystals formed are {100} oriented, their grains size in the range of 5-8 µm (Fig. 1); the roughness is about 0.4 µm, and the coating thickness 10 µm. EDS analyses indicate that the films are composed only of carbon (no contamination), and local Raman spectroscopy shows


there is no graphitic structure in these coatings (see Figs. 3-20).

Figure 1: SEM image of a diamond coating obtained by the combustion flame process 2.1.3 Experimental conditions Experiments were carried out in air, at ambient temperature and 200°C during 30 minutes. Only the disc was heated. When the friction started, contact occurred between the heated disc and the diamond coating whose temperature was ambient. The sliding speed was set at the constant value of 0.1 m/s. Two normal loads were applied: 2 and 5N; a third load (7N) was used in addition at 200°C. Both the friction zones and the transfer were characterised by SEM, and analysed by EDS and local Raman spectroscopy. 2.2 Milling test Under semi industrial conditions milling test were performed using standardised, commercial tungsten carbide tool coated with polycrystalline diamond (SDHT 1204 AEFN ALC), under dry friction conditions. The character of the coating is shown at Fig. 2. The work piece was a commercial magnesium alloy (AZ91). The cutting parameters were: - cutting speed: 330 m/min - feed/tooth (fz): 0.2 mm - depth of cut (a): 1 mm - length of the cutting path during one revolution: 78

mm - cycle number: 105

Figure 2: The character of the polycrystalline diamond coating on the commercial cutting tool The cutting tools were analysed only from the point of view of the nature of changes produced due to mechanical loading during machining. Their dependence on temperature as well as the evolution of the wear of a function of time were not investigated. 2.3 Experimental results and discussions The friction coefficients obtained at the end of the pin-on-disc tests, for the various experimental conditions are summarised in the following table. Table 1: Steady-state friction coefficients of the diamond/WC couples, after sliding at 20 and 200°C, under the normal loads specified 2N 5N 7N

20°C 0.55 0.58 200°C 0.61 0.63 0.65

Neither from the variation of the friction coefficient, nor from the SEM images could we not observe any changes in the diamond coating. All diamond coated pins were analysed before and after the friction with Raman microscopy (Figs. 3-5). An area of 60x60 µm was scanned. The evaluation of the Raman spectra given by 488 nm wavelength blue laser was achieved, after curve fitting, by analysis of the following peaks: - sp3 hybridized diamond (1333 cm-1), - sp2 hybridized amorphous carbon ( “D” peak, ~1350

cm-1), - the diamond accompanying, on the diamond surface

being sp2 trans-polyacetylene rings (~1150 and ~1450 cm-1) and chains (~1524 cm-1),

- as well as the sp2 hybridized graphite (~1580 cm-1) peaks,

- furthermore using both the half peak widths and peak area.


Raman spectra of the polycrystalline diamond coated pins used at 20°C with 2 N normal load:

Figure 3: before friction test (as deposited)

Figure 4: after friction test (at 20°C and 2N)

Figure 5: Comparative representation of Raman-spectra at 20°C and 2N Comparison of the Raman spectra shows that at 2 N normal load practically no change was produced except that the trans-polyacetylene rings and chains on the diamond crystal surfaces were rearranged causing a small increase of intensity of the ~1450 cm-1 peak.



Figure 7: after friction test (at 20°C and 5 N)

Figure 8: Comparative representation of Raman-spectra at 20°C and 5N At the higher normal load the intensity of the amorphous carbon peak indicates that a small-scale, local amorphisation of the coating takes place.





Figure 11: Comparative representation of Raman-spectra at 200°C and 2N The comparative representation of the Raman spectra shows that the increase of the substrate temperature did not result in any important effect, except the half peak width of the amorphous carbon peak has decreased.




Figure 14: Comparative representation of Raman-spectra at 200°C and 5N Even at this temperature the increase of the normal load produced only a small, local amorphisation of the coating as shown by the increase of the amorphous carbon peak intensity.





Figure 17: Comparative representation of Raman-spectra at 200°C and 7 N Compared to the previous results, there appears to have been an increase in amorphisation, as well as graphitisation as revealed by the appearance of the graphite peak.

The results of the Raman analysis for the side of the edges of milling tips were evaluated under the same conditions as for the pins.

Figure 18: Raman spectrum of the coated milling tip before machining

Figure 19: Raman spectrum of the coated milling tip after machining

Figure 20: Comparative representation of the Raman-spectra of milling tip From the comparative representation of the Raman spectra we can conclude that only a small amorphisation of the coating has taken place.


SEM analysis of the coating on the edges of the tip shows that the coating has been destroyed locally, Fig. 21.

Figure 21: Split coating on the milling tool cutting edge The high resolution SEM image shows that the sphere-like crystal groups, which may be considered as an aggregation of single crystals, have not been split, but have been separated entirely from the surface of substrate. It is well-known that the nucleation of the diamond crystals during coating begins with the activation of carbon atoms at the surface. Practical substrates can be formed only from such materials, which contain carbon in large quantities. Accordingly, for polycrystalline diamond coatings a base layer cannot be used. In the case of tungsten carbide substrates (practically the only one substrate used for diamond coating), the cobalt content of the surface must be removed by etching in the interest of satisfactory strong C-C binding between coating and substrate. This cobalt poor, etched interface between the coating and the substrate constitutes the weakest component of the coated system. A further special character of polycrystalline diamond coatings is that during the grow of the crystals as well as after cooling, because of the different thermal expansion of coating and substrate, residual compressive stresses are produced in the coating. These residual stresses can be determined experimentally using the Raman shifts of the diamond peak [4].

S

w∆−=σ (1)

where: ∆w – difference of the theoretical and measured wavenumber of diamond peaks, S – elastic constant characteristic to the diamond, [2.16 cm-1GPa-1]

3 Numerical modelling FEM-based numerical simulation leads to the determination of the stability limit for the contact, namely, the value of the maximum normal stress which can be applied prior to mechanical degradation of the coating. The contact geometry and material properties of the coated pins are used as input for these calculations. 3.1 Definition of the model The pin-on-disk contact, corresponding with the tribometer configuration used during the experimental work, is viewed as made up of two parts; the tungsten carbide disk on the one hand and the tungsten carbide pin coated with polycrystalline diamond on the other. In turn the pin is composed of three parts: - a 10 mm dia. tungsten carbide (6-9 %Co) cylinder

with a hemispherical end, - the hemispherical surface which, as a result of acid

treatment, has a reduced cobalt content (1-2.5 % after rediffusion at the deposition temperature) to a depth of 5 µm to promote the carbon-carbon bonding between tungsten carbide and diamond coating during deposition and

- the polycrystalline diamond coating (thickness 6-8 µm) whose characteristics depend on deposition conditions.

Figure 22: Specimens in the pin-on-disk tribometer. For successful 3D modeling of the contact, it is essential to have a geometrical definition of the surface and this is achieved here by 3D roughness measurements to define the surface topography [5]. Results for the {100} diamond coating are shown in Figs. 23 and 24.

WC-disc

Etched


Figure 23: Topography of the diamond coated pin (top view).

Figure 24: Minimum resolution needed for the characterization of the topography of a {100} oriented diamond coating.

By incorporating the measured roughness values in a CAD program, it is possible to describe the topography by a simple mathematical surface. For example, the digitalized surface for the {100} orientation is shown in original size in Fig. 26.

Figure 25: The digitalized surfaces of the {100} oriented diamond coating in original size.

The geometrical model of the pin for Finite Element Analysis is then constructed using the digitized topology of the diamond coating, Fig. 26. Since the measured surface topology is never symmetrical, axial symmetery cannot be used. Following from the description of the pin above, three different volumes are defined to take into account the different material response of the coating, the cobalt-depleted surface layer and the bulk tungsten carbide.

Figure 26: The meshed FE model for the pin. In constructing an appropriate mesh it should be remembered that the minimum mesh density must at least be equal to the mesh density of the parameter lines (see Fig. 24.) if the surface topography is to be represented faithfully, Fig. 26. The surface quality of the disk is an one order of magnitude better than the surface quality of the diamond coating. Therefore the disk was modeled assuming ideal geomerty.

Figure 27: The meshed FE model of the pin on disk contact. 3.2 Definition of the material properties The mechanical and thermal properties of the tungsten carbide pin and the cobalt-depleted surface layer are a function of cobalt content [6]. For the other mechanical and thermal properties we used data which are given on


the MatWeb-Material Property Data website. All material properties which were used for the FE analysis are represented in Table 1. The tungsten carbide pin, the disc and the diamond coating were considered as elastic materials. If during the calculations, the normal stress exceeded the yield stress of the steel disk, it was assumed to be perfectly plastic. Table 2: Material properties for the FE analysis

Tungsten carbide

disc (10% Co)

Tungsten carbide

pin (8 % Co)

Tungsten carbide interface (2.5%Co)

Diamond

Young’s modulus [GPa]

570

580

650

1143

Poisson’s Ratio 0.23 0.23 0.23 0.07

Constant of thermal expansion [µm/moC]

8

5.2

4.6

2.5

Conductivity [W/mK] 71 76 110 1500

Specific heat [J/kgoC] 700 700 700 3000

Density [kg/m3] 16000 15700 15000 3500

Contact heat transfer capacity (evaluated values) [W/m2K]

WC/ diamond (friction contact)

106

WC/WC interface

(glue)

108

diamond/

WC interface (glue)

107 3.3 Definition of initial and boundary conditions During the numerical analysis, the disk turned 360 degree in 360 steps while the pin was fixed, i.e., it could move only parallel with its axis and cannot rotate. The pin was mechanically loaded by a face load. A range of loads was used for Finite Element Analysis: 2, 5, 7 and 10 N. For the friction coefficient we used values which were measured experimentally from the pin on disk apparatus (Tab. 1). Thermal boundary conditions were defined: - the temperature of the upper surface of the disk was

set at, 20 oC or 200oC and treated as constant, - between the diamond coating and the tungsten

carbide interface, as well as the cobalt-depleted interface layer and the pin, a contact heat transfer was considered, assuming “glue” contact type between surfaces of different materials,

- between the disk and the diamond coated pin we assumed a contact heat transfer by friction only.

Following deposition of the diamond coating the average initial residual compressive stresses were measured by scanning Raman spectroscopy. These

values are shown in Table 2 and were used as initial condition for the numerical models. Table 3: Residual stresses in the diamond coatings of the pin, after deposition

Pins used for the given

tests

Measured wavenumber of diamond peak [cm-1]

Residual stress σx=σy [MPa]

2 N 1337.4 -2222 20°C

5 N 1333.6 -543 2 N 1334.1 -694 5 N 1333.8 -555

200°C

7 N 1334.6 -926 3.4 Evaluation of numerical results The numerical analysis of thermo-mechanical problems was performed as a series of coupled, quasi static problems. Fig. 28. and 29. show typical results of the FE analysis.

Figure 28: Equivalent stress (MPa) of the diamond coated pin (5 N; 20oC; 12th increment after the start of sliding).

Figure 29: The shear stress on the interface between the diamond coating and the Co-depleted WC layer (7N, 200oC, 8th increment after the start of sliding).


Numerical results were evaluated, on the one hand, as the maximum equivalent stress at the surface of the diamond coating (Fig. 28), on the other hand, as the maximum shear stress at the interface between the cobalt-depleted layer and the diamond coating, Fig. 29. As a first approach the shear stress developed during sliding at the interfacial cobalt-depleted layer was compared with the theoretical shear resistance of this layer. The shear resistance of the cobalt-poor interface layer can be determined using the classic relationship between critical shear stress and critical normal stress: 3/critcrit στ = (2)

In the case of tungsten carbide, the critical normal stress is the bending resistance, which also depends on the cobalt content, Fig. 30:

Figure 30: The bending resistance (σf) and the Rockwell-hardness (HRA) of hardmetals as a function of Co-content [6] Using eq. 2 and Fig. 30 the resistance of the cobalt-depleted layer can be determined. Taking the cobalt content as 2.5% and the bending resistance as 900 MPa, the shear stress limit is 520 MPa.

Figure 31: Calculated shear stress at the interface layer between tungsten carbide pin and diamond coating

By defining the calculated shear stresses on the tungsten carbide-diamond interface layer as a function of the normal load, the normal load at the onset of mechanical degradation can be determined with the help of the shear stress limit, Fig. 31. As a second approach, using 6900 MPa as compressive yield strength of the thin layers of polycrystalline diamond [8], we can determine the critical loads for cracking.

Figure 32: Calculated equivalent stress at the diamond coating Figs. 31 and 32 show that both delamination and cracking can occur at the same time. With increase of the contact temperature, the predicted critical load increased significantly. 4 Discussion As a result of crystal growth during deposition, as well as cooling following, residual stresses occur in the coating. Considering that the thermal expansion coefficient of the substrate is always higher than that of diamond, compressive residual stresses will be produced in the diamond coating. These compressive stresses were determined using Raman-spectroscopy (Tab. 3.). Under effect of increasing contact temperature the coating expands, e.g., a thermal stress with sign opposite to the residual stresses will be produced; therefore, the resultant stress state of the coating becomes more favourable for wear service. In consequence of this, the shear stress on the interface will also be smaller than at room temperature (Fig. 31.). If we disregard the residual stress due to crystal growth and assume that the residual stress is a consequence of the different thermal dilatation of the substrate and coating; we can determine an ideal service temperature for the diamond coating, at which the stress state, due to the thermal dilatation of coating, neutralizes the residual stresses due to the deposition [7]:

res

coat

coatdilat T

E σαν

σ =∆∆−

=1

(3)


( )0

1T

ET

coat

coatres +∆

−=α

νσ (4)

In the eq. 3 and 4: - ∆α is the difference of thermal expansion coefficient

of the substrate and the coating, - Ecoat is the Young’s modulus of the coating, - νcoat is the Poisson’s coefficient of the coating, - ∆T is the increase of temperature compared to the

room temperature, - T0 is the environmental temperature. Substituting in eq. 4 the corresponding values from the Tab. 2 as well as the measured residual stresses from Tab. 3, yields 360-400oC as the optimal service temperature range for diamond coated tools. 5 Conclusions Experimental investigations and numerical simulation were performed to determine the types of degradation of a polycrystalline diamond coating on a tungsten carbide surfaces. With pin-on-disc friction tests, using diamond coated WC pin against WC disc, we could not establish any mechanical degradation of the coating. Only a small scale amorphisation and, with increasing normal load, the onset of the graphitisation, could be detected by Raman analysis. In the case of a polycrystalline diamond coated tungsten carbide milling tip, which was used for semi-industrial milling tests, fracture of the coating was detected. The results of the Raman analysis of the coating on the sides of the cutting edges, before and after the machining, confirmed incipient amorphisation and graphitisation of the coating. Numerical simulation of the pin-on-disc test leads to determinating the stress and strain states of the coating during dry friction. Taking into consideration also the measured residual stresses of the coating, as well as the decreased resistance of the interface layer between the coating and the substrate, we could determine on the one hand, that both the delamination and the cracking can take place at the same time. On the other hand, we could demonstrate by calculation, that with increasing temperature the equivalent stress of the coating decreases, because of the decrease of residual stresses due to the thermal dilatation of the substrate. Acknowledgement This work was supported by the Hungarian Scientific Research Fund (OTKA) under project No. T 046587 and by a French-Hungarian bilateral agreement (TÉT F11/03).

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Technische Akademie Esslingen, Proceedings on CD-ROM, 15-F8