CLASSES OF MATERIALS
1.04 Cemented Tungsten Carbide HardmetaldAn Introduction
1.04.3 Mechanical Properties and the Role of Microstructure in Cemented WC .1 General Properties
to the target and/or tolerance of the above-mentioned properties for the different sintered products, and the quality of the sintered products is basically controlled per these specifications.
1.04.3 Mechanical Properties and the Role of Microstructure in Cemented WC
still much higher than for many other widely used engineering materials, and in this system the Co significantly improves fracture toughness. The range of Co composition within the WC–Co system provides this material with its exceptional range in mechanical properties, and hence its broad utility.
Another very important difference between WC and other ceramic materials is that WC is an excellent thermal conductor, while most ceramics are thermal insulators. A less known fact is that the thermal conduc- tivity of WC is higher than for most metals, including Fe, Ni, and Co. Consequently, the higher the content of the Co binder phase in cemented carbides, the lower the thermal conductivity (Frandsen & Williams, 1991;
Percherla & Williams, 1988). Additionally, WC–Co shows a higher thermal conductivity than TiC–Co or mixed carbide systems (Frandsen & Williams, 1991). The high thermal conductivity of WC is due to the lower phonon scattering in the hexagonal crystal structure, which shows a low incidence of point defects (Frandsen & Williams, 1991;Percherla & Williams, 1988), whereas TiC with its cubic structure tends to have missing C atoms in the structure that increase phonon scattering at room temperature (Frandsen & Williams, 1991) thus, decreasing thermal conductivity. The superior thermal properties of WC play an important role in separating it from typical ceramic materials, opening applications that require good heat flow in addition to high hardness and/or modulus.
1.04.3.2 Engineering Properties and Dependence on Microstructure
The primary industrial applications of WC–Co put it in the category of structural materials. The most important mechanical properties of WC–Co include hardness, transverse rupture strength (TRS), fracture toughness, impact resistance, and wear resistance.Table 2 lists out the ranges of these properties for metal alloys and ceramics, and for typical WC–Co, considering cobalt contents from 3 to 30% by weight.
TRS measurements are sensitive to porosity and other defects, particularly in the near surface region. TRS can be measured according to ASTM B406, using a three-point bendingflexural strength test that puts the test surface under tensile stress. Since retained porosity in sintered WC–Co was often a limiting factor in the microstructure of these materials and how they would respond to various applications, TRS was used extensively in evaluating these materials during their development. The test continues to be widely used as a quality control indicator today despite the fact that porosity levels have dropped significantly in the commercial grades of WC–Co due to the advances of modernized manufacturing processes. Historically, cemented carbides tended to have a large scatter in TRS data (Exner & Gurland, 1970). With appreciable porosity, TRS and fracture toughness tend to track one another closely, since the pores act as defects of a critical size for fracture toughness, governed by KIC¼Asr(ac)0.5, whereAis a geometrical constant, sris the rupture strength andacis the defect or pore size.
With negligible porosity, however, the static tensile state in TRS measurements can be more contingent on the intrinsic mechanical properties of the WC–Co system.
Detailed examination has revealed a complex relationship between TRS, hardness, and fracture toughness, even though TRS has sometimes been regarded as having a simple inverse relationship with hardness. An examination of samples with various Co contents and grain sizes, but excluding the hardest samples with very low Co or submicron grain sizes, showed a bell-shaped curve of TRS versus hardness with maximum TRS values occurring between hardness values from 1100 to 1300 kg mm2(Fang, 2005). The same study showed a high scatter of very hard samples, correspondingly with a very low fracture toughness, at hardness values
>1600 kg mm2. The relationship between TRS and hardness can also be analyzed by examining its depen-
dence on cobalt content and grain size separately. TRS values are highly contingent on Co content with a maximum value at approximately 20 wt% Co.Exner and Gurland (1970)also showed variation of TRS with WC grain size, again with a bell-shaped curve, and with a maximum TRS in the WC grain size range of 3–4mm.
In terms of fracture toughness, cemented WC is generally considered a brittle material, and the primary failure mode for most tool applications involves chipping or fracturing. In order to minimize and eliminate the
Table 2 Comparison of ranges for mechanical properties of WC–Co with those of ceramics and metal alloys Material Hardness (HvN mm1) TRS (MPa) Fracture toughness (MPa ffiffiffiffi
pm
) Impact resistance Wear resistance
WC–Co 1000–2300 2000–4000 9–20 Moderate High
Ceramics 500–2000 3–7 Low Moderate
Metal alloy 300–900 <5000 20–100 High Low
possibility of catastrophic failure during service, fracture toughness (KIC) of WC–Co must be considered a key mechanical property, dictating the selection of different grades of WC–Co materials.
A large body of literature is available on the fracture toughness of cemented WCs:Peters (1979),Bolton and Keely (1982), Sigl, Exner, and Fischmeister (1986), p. 631, Sigl and Exner (1987), Ravichandran (1994), Luyckx, Sacks, and Love (2007), and Zhang, Fang, and Belnap (2007). A. Shatov also further addresses an examination of fracture toughness within this volume in Chapter 1.10. Fracture toughness differs from TRS in that it is essentially a measurement of the materials resistance to crack propagation. It is well established that the fracture toughness is inversely proportional to the hardness of the material as shown in Figure 6, and both hardness and toughness are functions of the cobalt content and grain size of the material. For a given grain size, KICincreases with cobalt content, and for a given Co%,KICincreases as grain size increases (Luyckx et al., 2007).
The combined effects of grain size and cobalt can be expressed by the dependence ofKICon the mean free path (MFP) between WC grains, which is effectively the size of cobalt pools between WC grains.Figure 7shows the dependence of both hardness and fracture toughness on MFP. MFP is related to grain size and cobalt content by the following equation:
l ¼ 4ð1VVÞ SV
wherelis MFP,VVis the volume fraction of WC particles, andSVis the total surface area of WC grains which is determined by the grain size as well as cobalt content.
In the WC–Co system, the dependence of fracture toughness on microstructure is indicative of the tough- ening mechanisms in cemented carbides by the addition of Co. Sigl et al. systematically studied the propagation
8.00 12.00 16.00 20.00
800 1000 1200 1400 1600 1800
Hardness, Hv (kg mm−2) Fracture toughness, KIC (MPa m1/2)
Figure 6 Fracture toughness versus hardness for a range of cemented carbide compositions.
(a) (b)
Figure 7 Mean free path for a range of cemented carbide compositions plotted against (a) hardness and (b) fracture toughness.
Cemented Tungsten Carbide HardmetaldAn Introduction 129
of cracks in a WC–Co composite and found that 90% of the energy consumed during fracture of WC–Co is attributable to the plastic deformation of the cobalt phase in the path of the crack tip movement (Sigl et al., 1986, p. 631). The ductile tearing and crack bridging of cobalt is the primary mechanism for toughness in the WC–Co composite microstructure.
In testing procedures for fracture toughness, it is generally necessary to introduce sharp preexisting cracks of a known length to the samples before testing. Due to the principally brittle nature of cemented carbides, the introduction of precracks is not trivial, although a number of techniques have been successfully developed (Bolton & Keely, 1982). Consequently, TRS experiments are favored over fracture toughness in some cases, although the two tests do not investigate precisely the same materials phenomenon.
Early analyses of the plastic deformation process in WC–Co proposed that the ductile Co phase was completely responsible for plasticity in this system, and that the WC particles acted solely as dispersion strengthening agents (Doi, Fujiwara, & Miyake, 1969). Other researchers have noted the contribution of WC deformation in relation to the deformation of the composite structure:Takahashi and Freise (1965),Hibbs and Sinclair (1981),Chernyavsky (1986), andSarin and Johannesson (1975). Chernyavsky noted contributions from the slip within WC grains, as well as the slip along intercarbide grain boundaries.
Damage evolution has been investigated using a Hertzian indentation technique, which uses a near frictionless spherical indenter loaded normal to a polished surface (Zhang, Fang, et al., 2007). In addition to obtaining stress–strain curves in this work, Zhang measured and evaluated surface and subsurface microscopic and macroscopic cracking. The material response was categorized into three regions with increasing stress: a pseudoelastic region with an initial linear stress–strain response; a quasiplastic zone characterized by Co-binder deformation, as well as slip within WC grains and evidence of microcracking; and a fully plastic zone where the stress–strain curve reaches a plateau and the skeleton of interconnected WC grains is broken (Zhang, Fang, et al., 2007).
1.04.3.3 Wear Resistance
Owing to its very high hardness and strength, cemented WC has excellent wear resistance, which is the primary reason why WC–Co is chosen for most applications. For any given wear environment, the mechanisms of wear loss may be different, and the wear resistance of the material may vary. The main wear applications include 1. Adhesive and chemical wear during metal cutting
2. Abrasive wear during rock drilling
3. Sliding wear when used as machine rotational components
Similar to other materials used for wear applications, the wear resistance of WC–Co is determined by three categories of factors: mechanical properties, microstructure features, and the wear environment. The most important and primary factor that determines the wear resistance of WC–Co is the hardness.Figure 8shows the dependence of high stress abrasive wear resistance of WC–Co as a function of hardness as measured by the standard method prescribed in ASTM-B611.Figure 8also shows that wear resistance is inversely proportional to
0 5 10 15 20 25 30
5 10 15 20
84 86 88 90 92 94
High stress abrasion wear No.
(ASTM B-611), 1000 rev cm−3
Fracture toughness (KIC), MPa m
Hardness, HRa KIC
Wear No
Figure 8 Wear resistance and fracture toughness versus hardness for WC–Co materials. AfterFang (1998).
the fracture toughness, which is directly a result of the inverse relationship between the hardness and the fracture toughness.
The same trend also applies to other types of wear environments. However, in addition to hardness, other factors must also be considered, such as mechanical loading conditions, wear mechanisms, harshness of abrasive particles, and hardness and size/shape of abrasive particles. In cases of abrasive wear, the relative size between the hard phase particles,dc, and abrasive particles,da, is an important factor. In general, ifdc<da, the rate of wear is greater than it would be ifdc>da. In cases of sliding wear, relative hardness between the two mating surfaces, the chemical affinity between the two mating surfaces, and abrasive particles contained in the materials are all important factors that affect the wear behavior and wear rate. Depending on the wearing system, different mechanical properties (e.g. resistance to plastic deformation, crack formation, or crack propagation) have to be considered. Depending on which wear mechanism is dominant, different material properties and microstructure may be desired. Other environmental factors such as corrosive fluids and potential chemical interactions need to also be considered. Chapter 1.12 by M. Gee sheds further illumination on the mechanisms and effects of wear in this system.
1.04.3.4 Interrelationship between TRS, Hardness, Wear Resistance, and Fracture Toughness and Comparison with Ceramics and Metals
The interrelationship between hardness, wear resistance, and fracture toughness is, in general, consistent with the fundamentals of structural metal alloys. The relationship between TRS, hardness, and fracture toughness is, however, not straightforward.
Historically, the industry has considered TRS as a measure of the“toughness”of the material. The perceptions are that the higher the cobalt content or the lower the hardness, the higher the TRS. Fang examined the re- lationships between TRS, hardness, and fracture toughness and found that the relationship between TRS and hardness is not a monotonic one as was perceived (Fang, 2005).Figure 9shows that TRS decreases as hardness increases from HRa 87 to 92, but, it increases as hardness increases from HRa 85 to 87. When hardness is>HRa 93, the corresponding TRS values increase dramatically. In recent years with the advent of commercial ultrafine grain-cemented WC products with hardness equal or greater than HRa 93, TRS values are typically reported to be 4000 MPa. This is understood on the basis that TRS is essentially a tensile strength property. As hardness increases, the yield strength of the material increases proportionally. However, TRS is sensitive to defects, including microcracks. In the medium hardness range, the fracture toughness becomes the dominating factor;
hence, TRS decreases as hardness increases and toughness decreases.
The relationship between hardness, fracture toughness, and wear resistance (Fang, 1998, pp. 965–977) is shown inFigure 8. Due to the inverse relationship between hardness and toughness, and the proportional relative relationship between wear resistance and hardness, the wear resistance and fracture toughness are inversely related. This is the trade-off between wear resistance and toughness that engineers must make when selecting a particular grade for a specific application.
The role of cemented WC tools in the industry is dictated by cemented carbides’ mechanical properties relative to those of metal alloys and ceramics. The performance of cemented WC is determined by a
2800 3200 3600 4000 4400
800 1000 1200 1400 1600 1800
Hardness, Hv (kg mm−2)
TRS (MPa)
Figure 9 Transverse rupture strength versus hardness for cemented carbides. FromFang, 2005.
Cemented Tungsten Carbide HardmetaldAn Introduction 131
combination of mechanical properties that set it apart from metal alloys and ceramics.Figure 10illustrates that WC–Co is equivalent to ceramic materials in terms of hardness. WC–Co, however, has a much higherflexural strength and ductility than traditional ceramic materials. In contrast, WC–Co does not have the exceptional ductility of metal alloys, but has a much higher hardness and strength. Therefore, WC–Co is a material that spans the spectrum of mechanical properties between ceramic and metals.Figure 11further shows the unique position of WC–Co relative to metals and ceramics when considering ductility and toughness. Once again, it is the unique combination of mechanical properties that has determined the specific applications and broad role of WC–Co in manufacturing industries; a range of applications that cannot be matched or replaced by any metal or ceramic material currently known.
1.04.3.5 The Role of Coatings
Approximately 50% of all sintered WC–Co materials are used as cutting tools for the machining of metals and other materials. Since the 1980s, and continuing through current use, the majority of WC–Co cutting tools are coated by a thin film of ceramic material, using either chemical vapor deposition (CVD) or physical vapor deposition (PVD) methods. Major PVD techniques include sputtering and evaporation methods, while CVD describes methods involving reactions between gases at the surfaces of heated substrates. While PVD coatings are applied under partial vacuum, CVD processes involve elevated temperature and pressure. The ceramic coating materials are typically carbides, nitrides, or carbonitrides of Ti, Cr, Al, Hf, or Ta, although aluminum oxide is also widely used and diamond coatings are becoming more readily available. Coatings of nano- composite microstructure are also being investigated (Chen, Du, Wang, Wang, & Xu, 2009;Wang et al., 2010).
The thinfilm coatings range from a few micrometers to several tens of micrometers. Often the thinfilm consists of multiple layers of nanometer thicknesses, with different alternating coating compositions. Generally, it is desired to select coatings composed of materials with a low chemical affinity to the materials being machined or worked, in order to minimize adhesion wear and favor abrasive wear as the primary wear mechanism. In Chapter 1.16 by U. Schleinkofer, the role of coatings is examined in further detail.
Figure 10 The relationship between hardness, ductility, andflexural strength; comparing metal alloys, ceramics and WC–Co.
Figure 11 Relative placement of ceramics, WC–Co, and metal alloys, showing the relationship between fracture toughness,KIC, versus ductility.
There are a number of advantages of coated WC–Co over conventional uncoated WC–Co materials, including higher wear resistance, potentially better chemical stability, and some coatings also provide higher potential tool speeds, which result in faster cutting or machining operations. Many of these attributes result in cost savings in manufacturing processes, which accounts for their high market share. Additionally, by main- taining dimensional integrity of the tools during operation (by virtue of the high wear resistance), greater dimensional accuracy can be obtained in difficult to machine alloys like Inconel 718, other high nickel alloys and Al6Ti4V. This is particularly useful in parts produced for certain aerospace applications, as well as other applications requiring high precision machining (Biksa et al., 2010;Fox-Rabinovich et al., 2010).
Cemented carbides provide a high modulus–high hardness substrate for these hard wear-resistant coatings, but care must be taken in both the design and fabrication of the coated materials in order to obtain maximum wear efficiency. Normally, it is desired to have coatings with similar thermal conductivity and coefficient of thermal expansion (CTE) to that of the cemented carbide. A large mismatch in CTE between the coating and substrate can promote loss of the coating due to high shear stresses generated at the coating–substrate interface, depending on the temperature range of the coating process or the temperature of the in-service application. This is among the reasons for multilayer coatings, where both intermediate layer thickness and composition can minimize residual shear stresses during temperature variation. Most coatings are also designed to be under residual compressive stress, which contributes to coating retention during use.
Coating retention is a major consideration in the design, production, and application of coated cemented carbides. To this end, cleaning of the surfaces is often done by ion etching, solvent, or chemical processes to prevent debonding or inadvertent porosity at the coating–substrate interface. Subsequently, surface pre- treatments are often employed in both PVD and CVD processes to modify the chemistry, morphology, or roughness of the WC–Co surfaces. These pretreatments are sometimes used in combination, and include mechanical, chemical, and thermal methodologies, as well as laser ablation and the application of interlayers (Arroyo, Diniz, & Fernandes de Lima, 2010). As an example, Bouzakis et al. recently investigated the effects of different nanointerlayers (W, Cr, and Ti deposited by high-power pulsed magnetron sputtering) for PVD coatings of AlTi(N) with varied surface roughness (Bouzakis, Skordaris, et al., 2010). Their results indicated that a Cr-adhesive nanolayer and a polished and microblasted surfacefinish gave the best tool life in this coating system. Because the microscopic stress states at the interface play an integral role in coating adhesion, the thickness of interlayers must be considered in relation to the roughness of the substrate surface (Bouzakis, Makrimallakis, et al., 2010). Both testing andfinite element analysis can be useful in such efforts to obtain the best coating adherence and, therefore, tool life.
A number of coating compositions with Vicker’s hardness values, as high as 8000HV, are being studied or have been introduced to the marketplace in recent years (Upadhyaya, 1998). These include TiB2, BN, TiBXNY, diamond, and diamond-like carbon.
1.04.3.6 Novel Microstructures
Over the past half century, great strides have been made in the science of cemented carbides to understand the relationships between the chemical constituents, and to improve processing technology. These efforts have resulted in low-porosity materials suitable for a wide variety of tool applications, and the advent of coatings has pushed their utility further still. The desire to increase wear performance without sacrificing fracture toughness, or to improve fracture toughness without decreasing wear resistance, has led to several innovative micro- structures, including functionally graded (FG) and double cemented carbides (DCCs).
DCCs describe a microstructure whereby granules of high WC content are dispersed in a continuous Co matrix, resulting in a much higher MFP within the Co matrix compared to conventional cemented carbides (Deng et al., 2001;Fang, Lockwood, & Griffo, 1999;Fang & Sue, 2005). The much larger regions of Co between granules serve to improve fracture toughness, while the hard granules support load during wear, thus resulting in a material that performed particularly well under conditions of high stress wear.
FG microstructures have been introduced in other composite systems, and involve a variation from overall chemistry at the surface, and a gradual change in chemistry moving away from the surface into the bulk of the material. This gradual change in chemistry results in a corresponding gradual change of material properties from the surface into the interior, and consequently avoids the discrete change in properties, as well as the level of residual shear stresses, encountered at the interface of many coated materials.
Although FG cemented carbides have been fabricated with excess Co at the surface (Nemeth & Grab, 1986), most efforts have been toward producing materials with higher concentrations of WC at the surface and a zone Cemented Tungsten Carbide HardmetaldAn Introduction 133