Properties of amorphous SiC coatings deposited on WC-Co substrates

Properties of amorphous SiC coatings deposited on WC-Co substrates

A.K. Costa; S.S. Camargo Jr

Engenharia Metalúrgica e de Materiais, Universidade Federal do Rio de Janeiro, C.P. 68505, 21945-970 Rio de Janeiro, RJ, Brazil

^{Address to correspondence}

ABSTRACT

In this work, silicon carbide films were deposited onto tungsten carbide from a sintered SiC target on a r.f. magnetron sputtering system. Based on previous results about the influence of r.f. power and argon pressure upon the properties of films deposited on silicon substrates, suitable conditions were chosen to produce high quality films on WC-Co pieces. Deposition parameters were chosen in order to obtain high deposition rates (about 30 nm/min at 400 W rf power) and acceptable residual stresses (1.5 GPa). Argon pressure affects the energy of particles so that films with higher hardness (30 GPa) were obtained at low pressures (0.05 Pa). Wear rates of the coated pieces against a chromium steel ball in a diamond suspension medium were found to be about half of the uncoated ones. Hardness and wear resistance measurements were done also in thermally annealed (200-800 °C) samples revealing the effectiveness of SiC coatings to protect tool material against severe mechanical degradation resulting of high temperature (above 500 °C) oxidation.

Keywords: silicon carbide coatings; tungsten carbide tools

1. Introduction

Coatings based on hard materials are specially suitable for the purposes of protection against metallurgical tools wear. The association of properties involving high hardness and thermal stability with low wear rates and friction coefficient is the main goal to achieve. Silicon carbide is a material that presents these features but have being quite unexplored mainly due to its sometimes poor adhesion to metallic substrates.

Thin films of silicon carbide and silicon-carbon alloys are of great scientific and technological interest since these materials present an outstanding set of properties like good mechanical resistance¹, high hardness² and very high thermal stability^1,3,4. Their applications may range from protective coatings against corrosion of steel^5,6 to microelectronic devices⁷ and from X ray mask materials⁸ to protection of thermonuclear reactor walls⁹, among others. These films can be deposited by a variety of techniques such as laser assisted deposition¹⁰, dynamic ion mixing¹¹, plasma enhanced chemical vapor deposition¹², magnetron sputtering² and many others. From the various possible choices, magnetron sputtering appears to be a very attractive one due to its relative simplicity, high attainable deposition rates and wide acceptance by industry. At the low temperatures (T < 500 °C) generally necessary for most applications SiC films are amorphous and can be produced with hardness comparable to that of crystalline SiC^3,13,14.

The so called hardmetals such as WC-Co are widely used as cutting tools material since their development in the 1920's due to their high hardness, strength and fracture toughness^15,16. However, machining conditions usually submit the contact area of the tools surface to temperatures in the range of 600 to 1300 °C^15,16,17, where the mechanical properties of cemented tungsten carbide are strongly degraded mainly due to severe oxidation¹⁶. Therefore, SiC is a promising material to be used as protective coating for WC cutting tools due to its outstanding thermal stability and much higher hardness. SiC thin films also present a minimal thermal expansion coefficient mismatch when compared to WC-Co (5.3x10^-6vs. 5.4x10^-6 K^-1)¹.

In the present work we carried out an investigation of SiC films deposited by rf magnetron sputtering with the aim of developing a material for application as metallurgical and protective coatings. In previous works mechanical (hardness and stress) and microtribological properties of the films and their relation to the deposition parameters were determined^3,14. From those results suitable conditions were chosen to produce films with hardness values at least equivalent to that of crystalline SiC (26-28 GPa)¹⁸ at a reasonably high deposition rates. This could be achieved at high enough applied rf power and low argon pressures. Additionally, under such conditions the smoothest surfaces with the lowest friction coefficients were obtained¹⁴.

2. Experimental

Silicon carbide films were deposited with a rf magnetron sputtering system (US Gun II) from a 3" sintered, commercial grade SiC target using pure argon as sputtering gas. Commercial polished WC-Co cutting tools and single crystalline Si (100) were used as substrates and placed on a unheated sample holder at about 7 cm from the target. Hardmetal substrates were cleaned by alkaline degreasing followed by ultrasonic bath and in situ sputtering. Selected deposition conditions were: argon pressure P = 5.5 × 10² Pa, rf power P_RF = 400 W, substrate bias V_B = -30 V and no intentional substrate heating. Films were produced with thickness of around 5

^-5

A precision ball-crater apparatus was used to perform both thickness measurements and abrasive wear resistance tests. It helped also to give a visual evaluation of adhesion of the coatings, since poor adhered coatings produced detached grains that left deep scratches on the spherical craters. Tests were performed with a 30 mm diameter chromium steel bearing rotating at 50 rpm with a 0.1

3. Results and Discussions

In

Table 1^1,18

In order to investigate direct effects of thermal annealing upon mechanical properties of SiC films with a minimal influence of oxidation, samples deposited onto silicon substrates were annealed in vacuum. As shown in

Fig. 1Fig. 1

Hardmetal samples were annealed at high temperatures in air and inspected in an optical microscope. Loose oxidation products (a greenish powder) were observed on uncoated WC-Co surfaces for annealing temperatures of 600 °C increasing in thickness for higher temperatures. In

Fig. 2a2bFig. 2cFig. 2d

Figure 3 shows hardness values for coated and uncoated samples after thermal annealing. In close agreement with the results obtained for silicon substrates, hardness of coated surfaces was found not to be affected by annealing. On the other hand, the hardness of the uncoated tools is strongly degraded at temperatures higher than about 500 °C, reaching to one sixth of the original values at 700 °C. The observed degradation of mechanical properties of uncoated WC-Co tools is due to severe surface oxidation of this material that results in an oxide layer with reduced hardness.

Abrasive wear rates of annealed coated and uncoated samples are presented in

Fig. 4Table 1_MAX

4. Conclusions

High quality silicon carbide films were successfully deposited on tungsten carbide cutting tools. Coated substrates were found to be twice as hard and wear resistant as uncoated ones. Upon thermal annealing severe oxidation was found to occur on the surface of uncoated WC-Co at temperatures over 500 °C leading to degradation of its hardness and wear resistance. No signs of oxidation were found to occur in case of coated tools so that mechanical properties remained unchanged up to 700 °C. At higher annealing temperatures, however, the WC-Co oxidation caused the films to peel off from substrates. Annealing of films deposited on silicon substrates showed, however, that the coatings properties may remain unaffected up to 1000 °C.

Acknowledgements

This work was supported by CAPES and CNPq Brazilian agencies. The authors are also grateful to Sandvik do Brasil which kindly supplied the WC-Co cutting tools.

References

S.S. Camargo Jr

e-mail: [e-mail]

Received: January 02, 2002

Revised: September 30, 2002

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Publication Dates

• Publication in this collection
  25 Mar 2003
• Date of issue
  Jan 2003

History

• Received
  02 Jan 2002
• Reviewed
  30 Sept 2002