Research Paper
In situ measurements of local temperature and contact stress magnitude during wear of ceramic-on-ceramic hip joints

https://doi.org/10.1016/j.jmbbm.2013.01.018Get rights and content

Abstract

Fluorescence microprobe spectroscopy was applied to in situ assessments of contact stress and local temperature at the contact point of dry-sliding couples during wear tests of two commercially available ceramic-on-ceramic femoral heads. The investigated ceramic hip implants consisted of either monolithic Al2O3 or Al2O3/ZrO2 composite. A specially designed pin-on-ball tribometer was employed, which enabled directly testing the femoral head components as received from the maker without further manipulation. The strong fluorescence emission from Cr3+ impurities contained in Al2O3 served as a responsive sensor for both temperature and stress. Analytical corrections for the averaging effects arising from the finite size of the laser probe were made according to a probe response formalism in which geometrical conditions of the sliding couple were incorporated as boundary conditions. The sample-probe interaction at the contact point was then experimentally calibrated by obtaining probe response functions for the two materials investigated. Based on such theoretical and experimental procedures, deconvolutive computational routines could be set up and the true variations of local temperature and stress at the contact point of the bearing surfaces retrieved from the observed time-dependent broadening and shift of a selected spectral band, respectively. The main result of the in situ investigation was that the monolithic sliding couple showed both significantly lower temperature and lower magnitude of compressive stress at the contact point as compared to the composite one, although the composite couple wore at a significantly lower specific wear rate than the monolithic one.

Introduction

In vitro wear testing according to various tribological methods has been long and widely applied to the characterization of frictional forces and wear rates in load-bearing materials (Nair et al., 2009, Korres and Dienwiebel, 2010, Zemzemi et al., 2008, Patton and Zabinski, 2002, Kerkwijk et al., 1999, Cherif et al., 1997), as well as directly on hip joints by means of hip simulators (Saikko, 2005, Goldsmith and Dowson, 1999, McKellop and D'Lima, 2008, Ramamurti et al., 1998). Such approach not only enables estimating the wear resistance of the sliding materials but also provides important hints about the expected in vivo longevity of the implant. Nowadays, various kinds of tribometer device are available, which are routinely used to characterize the tribological properties of materials used in a variety of technologies. Among the available devices, the most basic equipments involve a flat or spherical surface that is driven by an external weight to move repetitively across the face of the tested material. The latter case corresponds to the classic pin-on-disc frictional test, as applied by Kerkwijk et al. (1999) for the evaluation of friction and wear characteristics of alumina and zirconia ceramics.

During wear testing, the occurrence of contact stress and the generation of heat at the contact area is inevitable. The sliding contact might cause quite severe temperature/stress gradients in the vicinity of the contact point (or surface) and represents the origin of structural degradation at the material surface. In current tribological assessments, precise but conspicuously phenomenological comparisons of the overall damage resistance can be made among different materials, but the physical, chemical and mechanical states of the material surface at the contact point are usually not explicitly represented in the monitored parameters. Some attempts have indeed been made to improve this situation. In the field of ceramic materials, Cherif et al. (1997) investigated the wear behavior of alumina–zirconia composites and measured the temperature in the mid-thickness of the disk sample during tribological test using a K-type thermocouple, which was fixed by means of alumina cement in a hole drilled in the disk. Unfortunately, this method was far from leading to accurate results, as far as the actual temperature value at the contact point between sliding surfaces was the target of the investigation. Nevertheless, this in situ attempt was conceptually important because it emphasized the possibility of expanding the outputs of conventional wear measurements from a merely phenomenological to a more physically insightful level. In the specific context of this paper, we notice that an improved in situ approach could play a quite relevant role in tribological testing of ceramic-on-ceramic bearing biomaterials, the most recent protagonists in the new generation of low-friction materials for arthroplastic applications (Piconi et al., 2003). However, for unfolding the local state of the material in situ at the contact point in such an important class of advanced materials, screening with high spatial resolution is needed. Therefore, a quite focused and responsive sensing mechanism is required. Succeeding in such a difficult task would contribute to comprehensively unveil not only the dynamics but also the inherent physics of the complex ensemble of wear phenomena.

Photo-stimulated spectroscopy can be regarded as a viable and quite powerful tool for realizing advanced tribological assessments. Shift and broadening of photo-stimulated spectral bands can bring direct and quantitative information about local temperature and stress fields, and other microstructural features occurring during wear of ceramic biomaterials such as alumina and zirconia. In situ tribological tests combined with fluorescence microprobe spectroscopy have a potential for fully satisfying all the requirements for an effective evaluation of the wear phenomena occurring at the contact point. However, as far as the geometry of a pin-on-disk tribometer is concerned, a direct application of photo-stimulated spectroscopy with the tribometer being placed under the optical microscope for in situ investigations could be problematic. In order to focus the laser at the contact point between the sliding surfaces, the pin (or the disk) should be made of a transparent material (e.g., glass or sapphire (Joly-Pottuz et al., 2007)). By doing so, however, the characteristics of the sliding couple become altered to a large extent, and thus the obtained tribological results cannot directly reflect the wear behavior of the actual sliding couple under evaluation. In other words, a different approach is needed for practical in situ studies of wear damage in biomedical sliding couples.

In this paper, we present in situ assessments of contact stress and local temperature at the contact point (or area) during sliding wear of ceramic-on-ceramic hip joints, as obtained in a specifically designed pin-on-ball tribometer by means of fluorescence microprobe spectroscopy. Leaving aside the full details of the new tribometer equipment, which are extensively given in a companion paper (Puppulin et al., in press), here, two different ceramic-on-ceramic hip couples (i.e., monolithic alumina and alumina/zirconia composite) widely used for hip joint replacements are investigated and compared in order to demonstrate the feasibility of this newly proposed spectroscopy-assisted tribology test. The selected sliding geometry and the use of a long-focus objective lens enables focusing, through precise calibrations, the incoming laser exactly at the contact point, while collecting the excited spectral signals from a relatively narrow probe volume. However, a serious complication arises from broadening of the fluorescence probe below the contact point from both sides of the sliding counterfaces. In order to solve this problem, the sample-probe interaction at the contact point was systematically analyzed from both theoretical and experimental viewpoints, and algorithms given to quantitatively describe a probe response function for the two studied sliding couples. Accordingly, probe averaging effects in space were removed through deconvolutive computational routines, and the true variation of local temperature and stress at the contact point retrieved within a degree of precision from the time-dependent broadening and shift of selected spectral bands.

Section snippets

Pin-on-ball tribometer for in situ photo-stimulated spectroscopy

Fig. 1 schematically shows the geometry of the specially designed tribometer, which enables wear tests with concurrent in situ investigations by fluorescence spectroscopy. A full description of the tribometer is given in a companion paper (Puppulin et al., in press). Briefly, the tribometer consists of a fixed pin and a rotating femoral head, brought into contact by an externally applied controlled load. The all tribometer device is placed under an optical microscope equipped with a

Probe response function for the sliding couple

In a microprobe spectroscopic device, when the laser beam is focused at an arbitrary location P0≡(x0,y0,z0) on the sample, the intensity of the observed fluorescence spectrum, Iobs(ω), is convoluted within the volume of the probe and arises from a plethora of individual spectra scattered from individual locations, P≡(x,y,z), comprised within the probe volume. Individual intensity contributions to the emitted signal are weight-averaged through a probe response function (PRF), namely, the

Determination of probe response functions for composite and monolithic sliding couples

The first step needed to unfold property gradients within the volume of a laser probe focused at the contact point is represented by the experimental determination of the PRF that characterizes the interaction between the incoming laser and the pin-on-ball system. To properly derive the probe parameters, defocusing of the laser along the sample depth was carried out at different locations of the femoral head by gradually shifting the laser focal plane along the direction of the incoming light,

Conclusions

In situ assessments of contact stress and local temperature at the contact point were carried out in a specially designed pin-on-ball tribometer during dry sliding of ceramic-on-ceramic femoral heads by means of fluorescence microprobe spectroscopy. Experimental investigations on two commercially available ceramic-on-ceramic couples BIOLOX®delta and BIOLOX®forte could be performed with taking advantage of the intense fluorescence emission of Cr3+ impurities contained in Al2O3. The sample-probe

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      However, metastability of the tetragonal phase, which as explained above actually represents a positive phenomenon in the toughening behavior of ZTA, could become a problem when it spontaneously takes place in-vivo at the bearing surface of the ZTA femoral head components. The possible negative consequences could be multifold: (i) an increasing fraction of monoclinic phase obviously reduces the remaining fraction available to phase-transformation toughening, (ii) some increase, although limited, in roughness of the surface might take place (Chevalier et al., 2009b); (iii) strong residual stress gradients can be introduced in the bearing surface and along the sub-surface, which will overlap the stress fields externally induced during service by both wear and body weight (Pezzotti et al., 2008; Tochino et al., 2006); and, (iv) the lower thermal conductivity of the gradually transforming monoclinic phase at the bearing surface leads to an increasing higher contact temperature upon frictional wear (Zhu et al., 2014). Typically, to predict the stability of the tetragonal polymorph in the ZTA femoral heads, materials scientists use an in-vitro simulation in hydrothermal environment.

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      All metal, ceramic, and ceramic coatings can be manufactured and polished to extremely fine surface finishes for articulation devices (i.e, <20 nm Ra). In vitro hip simulator studies, along with in vivo retrievals, demonstrate that polycrystalline ceramics or hard ceramic coatings are more scratch-resistant [150,258,476–478], typically have lower wear rates, and exhibit less osteolysis for self-mated bearing surfaces [247,479–489] and for articulation against polyethylene [150,476,477,490–499] than do CoCr implants. With less than half the hardness of ceramics, CoCr is particularly susceptible to third-body scratching.

    • Innovative tribometer for in situ spectroscopic analyses of wear mechanisms and phase transformation in ceramic femoral heads

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      However, the extent of transformation along the depth of the sample appears confined to a shallow region, because, unlike the confocal probe, the through-focus probe conspicuously fails in locating any amount of monoclinic fraction on the sample surface. The temperature increase measured in situ (i.e., concurrently to polymorphic transformation) from the linewidth of the chromophoric emission of alumina was found to be in the order of 134 °C (vs. 48 °C in the monolithic couple) and to decade very quickly (i.e., within about 2 μm) with distance from the contact point (Zhu et al., in press). Since the polymorphic transformation occurs with substantial volume increase, it can be one additional reason for the higher coefficient of friction measured in the composite couple (cf. Table 1).

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