1. Fracture toughness of carbides in tool steels evaluated by nanoindentation J. Caro a , D. Casellas a , S. Molas a , J. M. Prado a , I. Valls b a CTM Centre Tecnològic b ROVALMA S.A.

5. INTRODUCTION In order to develop high performance tool steels is necessary a detailed knowledge of the relationship between the hardness and fracture toughness of carbides and the macroscopic wear and fracture resistance of the bulk material The micrometric size of primary carbides to be analyzed forces the use of nanoindentation techniques Hardness and Young modulus : determined by confined indentation on carbides Fracture toughness : determined by the generation of cracks at the corner of the indentation This work has been recently published in D. Casellas et al., Acta Materialia 55 (2007) 4277 INTRODUCTION

6. In brittle materials fracture toughness (K C ) can be determined by the Indentation Microfracture Method (IM) Types of fracture generated by sharp indenters Halfpenny or radial cracks Connected underneath the hardness impression. The most common crack configuration for brittle materials Palmqvist cracks Developed at low indentation loads or in high-toughness materials (i.e. hard metals or zirconia ceramics) Berkovich cracks FRACTURE TOUGHNESS FRACTURE TOUGHNESS

7. Common expression used for K C determination (depending on indenter geometry and crack morphology): Anstis. : Limited to radial cracks. Applied to the symmetric indenters (i.e. Vickers) P indentation load, c crack length A = 0.016 and n =0.5 Laugier : Used for Palmqvist cracks x V = 0.015 Niihara : Used for Palmqvist cracks a : half-diagonal of the impression c = a + l  = 0.0089 (for WC-Co) [1] [2] [3] FRACTURE TOUGHNESS FRACTURE TOUGHNESS

8. FRACTURE TOUGHNESS FRACTURE TOUGHNESS Berkovich indenters are generally used in small-scale determination of K C . The most commonly used expression is Anstis equation. However, the nonsymmetrical nature of Berkovich indenters does not permit the radial cracks to be joined beneath the hardness impression The application of Anstis equation in this case is doubtful Dukino and Swain determined that good K c values using Berkovich indenters are obtained using Laugier expression with a little modification (x V = 0.016)

9. In this work, K C of micrometric primary carbides (10-20  m) in tool steels is determined using Berkovich indenters at low indentation loads (200-500 mN). It is expected to generate Palmqvist-type cracks (equations 2-3 are expected to give better results than equation 1). It is necessary to generate small cracks (shorten than 5-10  m) within the carbide. At this crack extension application of equations (1)-(3) could give rise to considerable differences in the calculated K C value. FRACTURE TOUGHNESS FRACTURE TOUGHNESS 9  m

10. Values of K C evaluated with expressions (1)-(3) for different primary carbides in different tool steels FRACTURE TOUGHNESS FRACTURE TOUGHNESS

11. K C values calculated by the expressions (1)-(3) at P=200 mN and a=2.8  m corresponding to the M 6 C carbides of 1.3202 steel. At short crack lengths Laugier’s expression (3) gives the highest values of K C , 2.4 times larger than Anstis’s expression (1) FRACTURE TOUGHNESS FRACTURE TOUGHNESS

12. As postulated previously, Laugier’s expression seems to be more appropriate for nanoindentation tests using Berkovich indenters. This expression has been successfully proved in a wide range of ceramic materials using Berkovich indenters. However, the differences between the results obtained with different expressions should be assessed. Consequently, the IM method was applied to a material whose K C is well known using low loads. The results were compared with the K C values obtained with macroscopic methods based on the propagation and fracture of large cracks reported in the literature. Chosen material  Silicon single crystal Si (100) FRACTURE TOUGHNESS FRACTURE TOUGHNESS

13. Nanoindentation on Si(100) plane at 200 mN Niihara (2) and Laugier (3) (using x V = 0.016) expressions give values of K C within the range of the values of Si reported with large crack testing methods FRACTURE TOUGHNESS FRACTURE TOUGHNESS large crack testing methods

14. Niihara et al. set the applicability of equation (2) within the range 0.4<a/l<1 In our case, almost all the data for the analyzed carbides lie outside this range FRACTURE TOUGHNESS FRACTURE TOUGHNESS

15. CONCLUSION: All these comments allow us to point out that the evaluation of K C by nanoindentation using Berkovich indenters should be conducted using Laugier expression (with x V = 0.016): This assertion is specially relevant at low applied loads, when cracks are shorten than 10  m, which usually occurs in nanoindentation of small particles such as in primary carbides of commercial cold work tool steels FRACTURE TOUGHNESS FRACTURE TOUGHNESS

17. RESULTS RESULTS Crystal structure was determined by XRD Samples were extracted with the same orientation, parallel to the rolling direction

19. RESULTS RESULTS catastrophic carbide fracture 20  m 20  m catastrophic carbide fracture

21. RESULTS RESULTS 1.3202 Steel M 6 C M 6 C MC 7  m 10  m 10  m

22. RESULTS RESULTS MC-II Steel B 15  m MC-I Steel A 9  m 8  m MC-III Steel B

23. RESULTS Fracture anisotropy in M 7 C 3 carbides in 1.2379

25. RESULTS RESULTS Arrows for M 7 C 3 in 1.2379 steel indicate that K C may be even lower Hardness and fracture toughness of different carbides Hard and tough carbides Potential candidates for incorporating in tool steels aimed at optimizing the relationship between wear and fracture resistance

26. RESULTS RESULTS Toughness (K C ) vs. composition of MC carbides V-rich carbides exhibit high values of K C