Keywords

1 Introduction

Co-Cr alloys have been established as metallic biomaterials essential for orthopedic, cardiovascular, and dental fields because of their excellent mechanical properties, high corrosion resistance, and high wear resistance [1–3]. Their wear resistance properties are particularly excellent as compared with those of other metallic biomaterials such as stainless steels and Ti alloys.

Pure Co undergoes an allotropic transformation at 690 K from the high-temperature γ-phase with the fcc (face-centered cubic) structure to the low-temperature ε-phase with the hcp (hexagonal close-packed) structure [4, 5]. This transformation is shear dominant with thermal hysteresis; therefore, it has been classified as martensitic [5]. The transformation temperature is changed by the addition of alloying elements to Co. In particular, biomedical Co-Cr alloys contain more than 20 mass% Cr, which improves the corrosion resistance by forming a passive layer consisting mainly of Cr oxide. The Co-Cr binary phase diagram is shown in Fig. 7.1 [6]. The addition of Cr increases the transformation temperature. The transformation temperatures in Co-20mass%Cr and Co-30mass%Cr alloys are around 1,100 and 1,200 K, respectively. Figure 7.2 summarizes the effects of alloying elements on the temperatures of the transformation from the hcp phase to the fcc phase of Co alloys [5]. The horizontal axis shows the change in the temperature of the hcp-to-fcc transformation from that for pure Co per 1at% of the alloying element, and the vertical axis shows the solubility of the alloying element in fcc Co. Alloying elements such as Fe, Mn, Ni, and carbon lower the hcp-to-fcc transformation temperature; they are fcc stabilizers. On the other hand, Mo, W, and Si increase the hcp-to-fcc transformation temperature and are hcp stabilizers, like Cr. This martensitic transformation is closely related to the microstructure and mechanical and chemical properties of the Co-Cr alloys.

Fig. 7.1
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Binary phase diagram of the Co-Cr system [6]

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Fig. 7.2
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Effects of alloying elements on the temperature of the transformation from hcp Co to fcc Co as a function of solubility of the elements in fcc Co [5]

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In this chapter, first, the history and current status of biomedical Co-Cr alloys such as Co-28mass%Cr-6mass%Mo and Co-20mass%Cr-15mass%W-10mass%Ni alloys are briefly reviewed. Their microstructure, processing, and properties are then discussed. Control of the microstructure of Co-Cr alloys is essential for practical applications, as it is for other metallic materials. Therefore, the effects of different processing techniques and the alloy composition including minor alloying elements on the microstructure are described with a focus on grain refinement and precipitation. As a novel processing technique for implant fabrication, an additive manufacturing technique using an electron beam and a laser beam is mentioned. Finally, the mechanical properties and corrosion and wear resistances of the alloys are presented. Hereafter, the chemical composition of the alloys is reported in units of mass%, although the mass% notation is omitted.

2 Co-Cr Alloys as Metallic Biomaterials

2.1 History

Co-based alloys in the Co-Cr and Co-Cr-W systems have been established since the early 1900s, and the Stellite alloys manufactured by Haynes became important industrial materials for wear-resistant hardfacing applications [5]. In the 1930s, a cast Co-Cr-Mo alloy, Vitallium, was developed, a technique for the lost-wax (investment) casting of the alloy was established, and the alloy was commercialized for dental prosthetics [7, 8]. Venable and Stuck used Vitallium devices for internal fixation of fractures and showed in the late 1930s that the alloy can be used in medical implants [9–11]. Vitallium was also used for prosthetic acetabular cups by Smith-Petersen in the late 1930s [12, 13]. These cups were used in the standard method for hip reconstruction until total hip replacement operations were introduced in the 1960s [12]. In the 1950s and early 1960s, metal-on-metal-type and metal-on-polymer-type artificial hip joints using Vitallium were developed. The former are represented by the McKee-Farrar joint, and the latter were pioneered by Charnley [14]. Other Co-Cr alloys such as Co-Cr-W-Ni (L-605, HS25, ASTM F 90) [15], Co-Ni-Cr-Mo (MP35N, ASTM F 562), and Co-Cr-Fe-Ni-Mo (Elgiloy, ASTM F 1058 grade 1) system alloys, which are still used in practical applications at the present time, had been developed in the early 1960s. Thus, the Co-Cr alloys have a long history consisting of more than 80 years of use as dental and medical materials.

Currently, cast and wrought Co-Cr alloys are widely used for implants such as artificial joints, denture wires, and stents. Metal-on-metal-type total hip replacements made of Co-Cr-Mo alloys were revived in the late 1980s [14] because the loosening of the metal-on-UHMWPE (ultrahigh molecular weight polyethylene)-type artificial hip joints was found to be related to osteolysis caused by the formation of UHMWPE wear debris [16]. Recently, however, a decrease in the number of stemmed metal-on-metal-type artificial hip joints made of Co-Cr-Mo alloys used for hip replacements in England and Wales was reported [17].

2.2 Types of Co-Cr Alloys

With their long history of practical applications, Co-Cr alloys have been listed in the ASTM standards for surgical implant applications [18]. Table 7.1 shows the chemical compositions from the standards. The contents of not only the main alloying elements such as Cr, W, and Mo but also minor alloying elements such as carbon and nitrogen are standardized. ASTM F 75 is for castings, and the others are for wrought products. The maximum carbon content in ASTM F 75 (Co-28Cr-6Mo) is 0.35 mass%. The low carbon content in ASTM F 75 alloy possibly causes the formation of the σ-phase (intermetallic compound, Co(CrMo)), which likely deteriorates the mechanical properties of the alloy. Carbon suppresses the formation of the σ-phase by forming carbides. The ASTM F 799 and F 1537 alloys, whose chemical compositions are registered as Co-28Cr-6Mo, have three types: Alloy 1 and Alloy 2 have low (C ≤ 0.14mass%) and high (0.15mass% ≤ C ≤ 0.35mass%) carbon contents, respectively, and Alloy 3 contains Al and La, which form oxide particles that provide dispersion strengthening [19].

Table 7.1 Chemical compositions (mass%) of Co-Cr alloys registered in the ASTM standards for biomedical applications
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Another classification of the Co-Cr alloys focuses on Ni content. The Ni content in ASTM F 75 alloy is required to be less than 0.5 mass%, and the Ni contents in ASTM F 799 and F 1537 alloys are required to be less than 1.0mass%. Other Co-Cr alloys such as ASTM F 90 (Co-20Cr-15W-10Ni), ASTM F 562 (Co-35Ni-20Cr-10Mo), and ASTM F 1058 (40Co-20Cr-16Fe-15Ni-7Mo) are allowed to contain Ni as an alloying element.

Figure 7.3 shows the stacking fault energies (SFEs) of three types Co-Cr alloys compared with those of Fe-based alloys [20]. The ASTM F 75 alloy, which is represented as Co-29Cr-6Mo in the figure, has a low SFE, so the hcp ε-phase with poor workability would form during the cooling process in a casting or heat treatment. In contrast, Co-30Ni-20Cr-10Mo (close composition to ASTM F 562) and Co-20Cr-15W-10Ni (ASTM F 90) alloys, which contain Ni as an alloying element, exhibit mid-level and high SFEs, respectively. Therefore, the formation of the hcp ε-phase is suppressed, and these alloys have excellent hot and cold workability. In this context, Co-28Cr-6Mo alloys are basically unsuitable for wrought applications, but the workability in the Co-28Cr-6Mo alloys can be improved by optimizing the amounts of minor alloying elements such as nitrogen and by thermomechanical treatments (see Sect. 7.4.1).

Fig. 7.3
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Temperature dependence of the calculated SFEs of Co-Cr alloys and Fe-based alloys [20]

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2.3 Applications

Table 7.2 summarizes the applications of Co-Cr alloys. Cast Co-28Cr-6Mo (ASTM F 75) alloy has been used in the stem, ball, and cup of artificial hip joints including both metal-on-UHMWPE and metal-on-metal joints as well as in the sliding components of artificial knee joints. Since artificial knee joints have complicated shapes, investment casting is essential for their production. Wrought Co-28Cr-6Mo Alloy 1 and Alloy 2 (ASTM F 799 and F 1537) have been used in the joint replacements for hips, knees, shoulders, and so on. The Co-Cr alloys in the Co-Cr-W-Ni (ASTM F 90/F 1091), Co-Ni-Cr-Mo (ASTM F 562), and Co-Cr-Fe-Ni-Mo (ASTM F 1058) systems are used for wrought products that require a large deformation to fabricate. Since they exhibit excellent hot and cold workability and can be strengthened by cold working, they have been used in fixation wires, vascular stents, springs, catheters, surgical clips, orthodontic dental archwires, and so on.

Table 7.2 Applications of Co-Cr alloys
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3 Microstructure and Processing in Co-Cr Alloys

3.1 Grain Refinement

Grain refinement can be used to strengthen metallic materials while maintaining their ductility. It is known that ultrafine grains smaller than 1 μm have been produced by severe plastic deformation; however, the severe plastic deformation techniques have not been applied to Co-Cr-Mo alloy parts because of their dual-phase microstructure and the strain-induced martensitic transformation during plastic deformation from the γ-phase to the ε-phase [21].

The grain refinement of Co-Cr-Mo alloys using conventional hot-compression deformation [21] and reverse transformation [22] has been reported. Yamanaka et al. [21] reported an ultrafine-grained microstructure with a grain size of around 0.6 μm obtained by the conventional hot-compression deformation of Co-29Cr-6Mo alloy with low carbon and low nitrogen contents. This microstructure was formed by dynamic recrystallization at 1,323 K under a strain rate of 0.1 s−1 from an initial average grain size of 40 μm. It was suggested that this fine microstructure was a product of the inhomogeneity in the local strain distribution induced by planar slips and deformation twins due to the lower SFE of this alloy at elevated temperatures [23]. A Co-29Cr-6Mo alloy with an average grain size of 0.8 μm produced by hot-compression deformation exhibited a significantly high 0.2 % proof strength of 1,330 MPa [24]. Kurosu et al. [22] developed a grain refinement process based on the reverse transformation from a lamellar (hcp ε-phase + Cr2N) phase to an fcc γ-phase for the Co-27Cr-5Mo-0.16N alloy. Their process consisted of a two-step heat treatment without a hot or cold plastic transformation: solution-treated alloy was subjected to isothermal aging at 1,073 K for 90 ks to form a lamellar structure consisting of the hcp ε-phase + Cr2N, and then the aged alloy with the complete lamellar microstructure was reverse-treated at a temperature of 1,273–1,473 K. The temperatures of 1,073 K and 1,273–1,473 K are in the stability regions for the ε -phase and γ-phase, respectively. The average grain size was reduced from 200 μm initially to 20–25 μm. Figure 7.4 shows the stress-strain curves of the Co-27Cr-5Mo-0.16N alloys after solution treatment (ST), aging treatment (AT), and reverse treatment (RT) at 1,273 K for 300 s [22]. The tensile strength was improved by the reverse transformation, while the ductility remained almost the same.

Fig. 7.4
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Stress-strain curves of the Co-27Cr-5Mo-0.16N alloys after solution treatment (ST), aging treatment (AT), and reverse transformation (RT) at 1,273 K for 300 s. A table of the mechanical properties for each specimen is shown as an inset in the figure (Reprinted from Ref. [22], Copyright 2010, with permission from Elsevier)

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3.2 Precipitation

Precipitates in the Co-Cr alloys are known to affect the mechanical properties [25, 26] and corrosion [27–29] and wear [30–34] resistances of both cast and wrought products. Therefore, knowledge of the precipitates present in as-cast alloys and after heat treatment is essential for establishing reasonable production processes for biomedical Co-Cr alloy devices.

The precipitates in Co-Cr alloys are classified into intermetallic compounds and carbonitrides [1]. Therefore, the carbon and nitrogen contents in Co-Cr alloys significantly affect the phase and morphology of the precipitates. Figure 7.5 shows the microstructures of as-cast Co-28Cr-6Mo alloys with carbon contents of 0 to 0.41 mass% and nitrogen contents of 0 to 0.24 mass% [35, 36]. A dendrite matrix and interdendritic and grain boundary precipitates were observed in the alloys, except in Fig. 7.5e. The amount of precipitates increased with the carbon content in the alloys. As shown in Fig. 7.5e, no precipitates were detected in the as-cast Co-28Cr-6Mo-0.20N alloy. In addition, no precipitates were formed in Co-28Cr-6Mo-0.20N and Co-28Cr-6Mo-0.25N alloys even after heat treatment at temperatures of 1,473–1,573 K for up to 43.2 ks, and the microstructure consisted of a single fcc γ-phase [36]. Thus, the formation of the σ-phase and the martensitic transformation from the γ-phase to the ε-phase can be suppressed by the addition of nitrogen to Co-28Cr-6Mo alloys without added carbon. Up to 0.25 mass% nitrogen is allowed in the ASTM F 75, F 799, and F 1537 standards.

Fig. 7.5
figure 5

Microstructures of as-cast Co-28Cr-6Mo alloys with various carbon and nitrogen contents: (a) 0C-0N, (b) 0.16C-0N, (c) 0.24C-0N, (d) 0.41C-0N, (e) 0C-0.2N, (f) 0.16C-0.2N, (g) 0.27C-0.24N, and (h) 0.34C-0.21N [35, 36]

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The morphologies of precipitates found in Co-Cr-Mo alloys are summarized in Fig. 7.6 [1]: blocky-dense morphologies (σ-phase, χ-phase, η-phase (M6X-M12X type), and M23X6 type), starlike-dense morphologies (π-phase), starlike morphologies with striped patterns (M23X6 type), starlike morphologies with complicated microstructures (including M7X3 type), and lamellar cellular colony morphologies (M2X type) have all been observed. In the notation for the precipitates, M refers to metallic elements such as Cr, and X refers to carbon and/or nitrogen. Clemow and Daniell [37] reported that the starlike shape was attributable to incipient melting around the edges of the carbide particles. The starlike shape of π-phase confirms that the π-phase forms under conditions where a liquid phase coexists. The M23X6-type precipitate contains little nitrogen, i.e., it is almost carbide [1], and it is coherent with the γ-phase because its lattice constant is around 3 times of that of the γ-phase. The M2X-type precipitate is a nitrogen-rich carbonitride that forms in nitrogen-containing Co-Cr-Mo alloys [1, 36, 38]. The η-phase [38] and π-phase [39] are carbon-rich carbonitrides, while the σ-phase [1] and χ-phase [1, 40] are intermetallic compounds. The formation of M7X3-type precipitates in the Co-Cr-Mo alloys was suggested by Kilner et al. [41] and was confirmed by the present authors [39]. However, the chemical composition of the M7X3-type precipitate has not been clarified, because its size was too small to be analyzed (see Fig. 7.6g). The alloying elements affect the stabilization of the precipitates: carbon stabilizes the M23X6-type and M7X3-type precipitates [39], nitrogen stabilizes the M2X-type and η-phase precipitates, and Si stabilizes the η-phase [42] and χ-phase [1] precipitates. The M23X6-type and η-phase precipitates were detected in Co-20Cr-15W-10Ni (ASTM F 90) alloy after heat treatment, although the as-forged alloy did not contain precipitates.

Fig. 7.6
figure 6

Morphologies and phases of precipitates observed in biomedical Co-Cr-Mo alloys: (a) σ-phase (blocky dense), (b) χ-phase (blocky dense), (c) η-phase (blocky dense), (d) π-phase (starlike dense), (e) M23X6 type (blocky dense), (f) M23X6 type (starlike with stripe patterns), (g) M7X3 type (starlike with complicated microstructures), and (h) M2X type (lamellar cellular colony) [1]

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The presence of the π-phase [35, 39] and χ-phase [40, 43] was revealed by the present authors. Figure 7.7a shows the X-ray diffraction (XRD) pattern of precipitates electrolytically extracted from as-cast Co-28Cr-6Mo-0.16C-0.13N alloy [36]. The main reflections of the precipitate were in good agreement with the pattern of a (Cr,Mo)12(Fe,Ni)8–xN4–z-type nitride (JCPDS card no. 26-0428) that was detected as a precipitate after aging of an Fe-25Cr-28Ni-2Mo-0.31N alloy [44]. This nitride phase is referred to as the π-phase [45] and exhibits a β-Mn structure. Its chemical composition can be ideally represented as M2T3X, where M and T are metallic elements with low and high affinities for X (carbon and/or nitrogen), respectively. Figure 7.7b schematically shows the region where the π-phase was detected after heat treatment of the Co-28Cr-6Mo alloys. The formation of the π-phase is confirmed at temperatures of 1,548–1,623 K, where partial melting occurs in the alloys. The formation of the π-phase under the partial melting conditions was observed in the Co-28Cr-6Mo alloys from almost all of the carbon content region (C ≤ 0.35 mass%) allowed by the ASTM F 75 standard. It was especially marked in the alloy with a carbon content of around 0.15mass%. Whether a π-phase precipitate is present in the alloys at room temperature depends on the cooling rate from the high temperatures at which the π-phase formed. Rapid cooling would be required to retain the π-phase precipitate in the alloys. A lower cooling rate seems to cause the π-phase to transform into other phases such as the M23X6-type + metallic fcc γ-phase.

Fig. 7.7
figure 7

(a) XRD pattern of precipitates electrolytically extracted from as-cast Co-28Cr-6Mo-0.16C-0.13N alloy and (b) schematic illustration of the formation region of the π-phase in Co-28Cr-6Mo alloys during heat treatment [36]

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The χ-phase has an α-Mn structure with 58 metallic atoms in the unit cell [46, 47]. The presence of the χ-phase as a precipitate in Fe-based alloys after isothermal heat treatment was reported previously [48, 49]. The χ-phase precipitate is considered to be harmful in Fe-based alloys because it is deleterious to their mechanical properties. As for Co-Cr-Mo alloys, the heat treatment periods and chemical compositions for which the χ-phase is formed are limited. Therefore, it is relatively easy to prevent the formation of the χ-phase during the fabrication of Co-Cr-Mo alloy devices.

3.3 Novel Processing for Implant Fabrication

Recently, additive manufacturing for metals has been paid much attention as a process for producing complex three-dimensional structures from precursor powders [50]. This technique can be used to make orthopedic implants optimized for each patient’s needs or skeletal structure. These custom-made implants are regarded as next-generation biomedical devices, and bone plates/screws, intramedullary nails, artificial joints, and so on are all expected to be fabricated using this technique [51]. Additive manufacturing using electron beam melting [52–54] and laser beam melting/sintering [55–58] has been performed to produce Co-Cr implants. In these techniques, CAD data for the implants are converted to data slices with heights of 20–100 μm. Using the slice data, an electron beam or a laser beam is irradiated onto a Co-Cr alloy powder with controlled particle size for melting and casting in a layer-by-layer manner (Fig. 7.8). Therefore, it is possible to precisely control the overall porosity and the size, shape, distribution, and direction of pores in the implants [59]. Electron beam melting is conducted under vacuum, while laser melting is operated under various atmospheres [60].

Fig. 7.8
figure 8

Schematic diagram of additive manufacturing using electron beam melting and laser beam melting

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Gaytan et al. fabricated fully dense cylindrical and orthogonal structures and knee implant components by electron beam melting using Co-26Cr-6Mo-0.2C alloy powder [52, 53]. They observed Cr23C6 carbide arrays with dimensions of 2 μm in the building plane perpendicular to the building direction, which formed carbide columns in the vertical plane parallel to the building direction [52]. Hot isostatic pressing at 1,473 K produced an equiaxed fcc γ-phase grain structure with some grain boundary carbides. The anisotropy of the microstructures and mechanical properties of the as-cast Co-28Cr-6Mo-0.23C-0.17N alloy products fabricated by electron beam melting was also reported [54].

Selective laser melting has been applied to Co-Cr alloy implants in the dental [57, 58] and medical fields [55, 56]. Takaichi et al. [57] examined the microstructure, mechanical properties, and metal elution of the Co-29Cr-6Mo alloy products fabricated with different laser powers and scan spacings. Dense structures were obtained for input energies higher than 400 J⋅mm−3, whereas porous structures were formed for input energies lower than 150 J⋅mm−3. They observed a microstructure with fine cellular dendrites in elongated grains parallel to the building direction. Furthermore, the fcc γ-phase was dominant in the fabricated structures, and its preferential <001> orientation was found to be along the building direction. The anisotropy of the mechanical properties due to the unique microstructure was also reported [57]. Finally, laser deposition of Co-Cr alloys was performed to add surface coatings for biomedical applications [61, 62].

4 Properties of Co-Cr Alloys

4.1 Mechanical Properties

Figure 7.9 summarizes the minimum requirements in the ASTM standards for the tensile strength and elongation of Co-Cr alloys. The mechanical strength and ductility of Co-Cr alloys can be changed over a wide range by controlling the microstructure using alloying and thermomechanical treatments, as is the case for other metallic materials.

Fig. 7.9
figure 9

Tensile strength and elongation required for biomedical Co-Cr alloys in ASTM standards

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The addition of small amount of alloying elements to cast Co-28Cr-6Mo (ASTM F 75) alloys is an effective way of controlling the microstructure. The addition of Zr [63] to ASTM F 75 alloys can yield a fine microstructure of the as-cast materials, resulting in increased tensile strength and elongation.

Nitrogen is an effective alloying element for improving the workability of wrought Co-28Cr-6Mo alloys [64], because it suppresses the σ-phase formation and γ-to-ε martensitic transformation, as described in Sect. 7.3.2. It has been reported that nitrogen stabilizes the γ-phase in the Co-29Cr-6Mo alloy by increasing the energy barrier for the γ-to-ε transformation and thus lowering the kinetic rate of the transformation by forming Cr-N short-range order [65] or nanoscale Cr2N precipitates in the γ matrix [66]. In fact, it has been reported that the addition of nitrogen improves the hot-working properties of the Co-28Cr-6Mo-0.16N alloy in the temperature range between 1,273 and 1,473 K [67]. The addition of nitrogen to Co-Cr-Mo alloys also increases their mechanical strength [68, 69]. It was reported [69] that strengthening of Co-29Cr-6Mo alloys was caused by the nanosized Cr2N precipitates in the γ matrix. Better elongation and workability were achieved in the alloy with a nitrogen content of 0.10 mass%, where the γ-to-ε martensitic transformation was completely suppressed. However, the further addition of nitrogen was reported to slightly decrease the elongation to failure because of the enhanced formation of annealing twins [69].

Carbide precipitation was reported to increase the tensile strength but decrease the elongation and workability of Co-Cr-Mo alloys, although these properties depend on the size and distribution of the precipitates.

Recently, an increase in the Cr content up to 34 mass%, which is above the ASTM F 75 composition range, was reported to increase the nitrogen solubility in Co-Cr alloys and improve the mechanical strength and ductility under as-cast condition [70]. Finally, a nitrogen-containing Co-33Cr-6Mo alloy with excellent ductility was reported to be promising for fabricating dentures with adjustable clasps using one-piece casting [71].

4.2 Corrosion Resistance

The excellent corrosion resistance of Co-Cr alloys is caused by the passive layer on their surface, which prevents further corrosion under biological conditions. The passive layer formed on Co-Cr-Mo alloy after polishing in pure water consisted of complex oxides of Cr and Co with a small amount of Mo oxides including hydroxyl groups, and its thickness was 2.5 nm [72, 73]. It is known that passive layers in contact with electrolytes undergo a continuous process of partial dissolution and reprecipitation from the microscopic viewpoint. Co was dissolved from the passive layer during immersion in Hanks’ solution and a cell culture medium and during incubation in a cell culture. In this way, a passive layer consisting of mainly Cr oxide with small amount of Mo oxide was reconstructed [73]. Hodgson et al. [74] investigated the passive layer of a Co-28Cr-Mo alloy in simulated biological solutions using X-ray photoelectron spectroscopy (XPS). They discovered that the major component dissolving from the alloy was Co and that the passive layer was mainly Cr(III) oxide along with a smaller amount of Cr(III) hydroxide.

Figure 7.10 shows the anodic polarization curves of as-cast and heat-treated Co-28Cr-6Mo-0.24C alloys in Hanks’ solution. The heat treatment was conducted at 1,523 and 1,623 K for 43.2 ks. The alloy heat-treated at 1,523 K had no precipitates, while the main precipitates in the as-cast alloy and the alloy heat-treated at 1,623 K were of M23X6 type and π-phase, respectively. The effect of the precipitates on the corrosion resistance of the alloys was not strong. However, Bettini et al. [29] suggested that M23X6-type and M6X-type precipitates affected the corrosion resistance of heat-treated Co-28Cr-6Mo-0.2C alloy on the basis of their microscopic surface observations after an electrochemical potential was applied. They observed that carbide boundaries were preferential sites for metal dissolution and that carbides with nonuniform compositions might exhibit different dissolution rates. Precise analyses of the microstructures of the precipitates and the chemical composition in the vicinity of the precipitates are required to understand the corrosion mechanisms of Co-Cr alloys under biological conditions.

Fig. 7.10
figure 10

Anodic polarization curves in Hanks’ solution for as-cast Co-28Cr-6Mo-0.24C alloy and for alloys heat treated at 1,523 and 1,623 K

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4.3 Wear Resistance

Excellent wear resistance is one of the characteristics of Co-Cr alloys. Such wear resistance is required in the sliding parts of artificial joints, for which Co-28Cr-6Mo (ASTM F 75, F 799, and F 1537) alloys are generally used. Therefore, the wear properties of the Co-28Cr-6Mo alloys are examined in this section.

As stated in Sect. 7.2.1, a decrease in the number of operations using stemmed metal-on-metal-type artificial hip joints made of Co-Cr-Mo alloys has been reported [17]. The main concerns regarding the use of metal-on-metal-type hip replacements are the formation of wear debris [75] and ion elution [76], which seem to be closely related to the microstructure of the Co-Cr-Mo alloy implants, which is influenced by the γ-to-ε phase transformation and precipitation of intermetallic compounds and carbonitrides.

Some studies have reported the effects of precipitation on the metal-on-metal wear behavior of Co-Cr-Mo alloys [32, 34, 77, 78]. Chiba et al. reported that marked abrasive wear was caused by precipitates such as σ-phase precipitates and that the Co-Cr-Mo alloys with ε-phase formed by strain-induced martensitic transformation during wear tests exhibited excellent wear resistance because of the increased hardness [34, 78]. Some studies have suggested that a high carbon content in Co-Cr-Mo alloys improves the wear properties [32, 77].

The present authors reported the pin-on-disk wear behavior in Kokubo solution for Co-Cr-Mo alloy pins with different precipitates and an Al2O3 disk [33]. As-cast and solution-treated (ST) Co-28Cr-6Mo-0.3N (0Si0Mn0.3N), Co-28Cr-6Mo-1Si-1Mn-0.3N (1Si1Mn0.3N), Co-28Cr-6Mo-1Si-1Mn (1Si1Mn0N), and Co-28Cr-6Mo-1Si-1Mn-3Ni (1Si1Mn3Ni0N) alloys were used as materials for the pins. The ST alloys had no precipitates, while the as-cast alloys contained π-phase, M23X6-type, and M2X-type precipitates, depending on the alloy composition. The mass loss of the pins after wear tests is shown in Fig. 7.11 [33]. The mass loss of the as-cast pins was higher than that of the ST pins. Figure 7.12 shows the surface morphologies of the as-cast and ST 1Si1Mn0.3N alloy pins after the wear tests [33]. The wear direction was from top to bottom in the figures. Continuous wear grooves were observed on the ST pin (Fig. 7.12b), while discontinuous deep wear grooves were detected in the as-cast pin after the wear tests (Fig. 7.12a). The formation of cavities and discontinuous deep grooves is likely to cause a higher mass loss for the as-cast pins than for the ST pins. Wear debris from both precipitates and the metallic matrix was found after wear tests of the as-cast pins. Some of the precipitates first detached from the metallic matrix, forming cavities during the wear tests, and then this debris caused discontinuous deep grooves through three-body abrasion.

Fig. 7.11
figure 11

Mass loss of the pins after a wear test in Kokubo solution (Reprinted from Ref. [33], Copyright 2013, with permission from The Japan Institute of Metals and Materials)

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Fig. 7.12
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SEM images of the surfaces of (a) as-cast and (b) ST pins of 1Si1Mn0.3N alloy after wear tests in Kokubo solution (Reprinted from Ref. [33], Copyright 2013, with permission from The Japan Institute of Metals and Materials)

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The amounts of eluted ions of each metallic element (Co, Cr, Mo, Si, Mn, and Ni) after the wear tests are shown in Fig. 7.13 [33]. The Co/Cr and Co/Mn mass ratios for ions eluted from the 1Si1Mn3Ni0N alloy pin were calculated to be 53.1 and 225.0, which are much higher than the ratios of 2.1 and 49.3, respectively, calculated from the alloy composition. This means that the Cr and Mn ion contents are lower than expected based on the chemical composition of the alloy. EDX analysis suggested the formation of calcium phosphate on the wear tracks of the disks. The calcium phosphate included Cr and Mn, indicating that the Cr and Mn ions were preferentially incorporated into calcium phosphate [33].

Fig. 7.13
figure 13

Amounts of eluted Co, Cr, Mo, Si, Mn, and Ni ions after wear tests in Kokubo solution (Reprinted from Ref. [33], Copyright 2013, with permission from The Japan Institute of Metals and Materials)

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Small amounts of Ni ions from Ni impurities in 0Si0Mn0.3N, 1Si1Mn0.3N, and 1Si1Mn0N alloy pins were detected after the wear tests. Decreasing the Ni content will be necessary to decrease Ni ion elution and thereby improve the safety of Co-Cr-Mo alloys.

5 Conclusions

The Co-Cr alloys have a long successful history of practical applications in the dental and medical fields since Vitallium was developed. It is impressive that Vitallium, or Co-Cr-Mo alloy, has been developed for dental applications and that the concept of Vitallium is still alive at the present time. The proportion of elderly people in the population is growing worldwide, and it is predicted that the number of patients suffering falls and deterioration of body functions will increase. Co-Cr alloys are seeing continued use in reconstruction devices for the human body, and the increases in their usage are expected. Co-Cr alloy has still much room for improvement, and research into topics such as microstructure control using newly developed production processes and the optimization of alloy composition using metallic and light elements is required, especially for contents beyond the ASTM standards.