Biomaterials, Volume 24, Issue 4, February 2003, Pages 619-627

Deposition of highly adhesive ZrO₂ coating on Ti and CoCrMo implant materials using plasma spraying

Yunzhi Yang^{, , a, b}, Joo L. Ong^{a, c} and Jiemo Tian<sup>b

a</sup> Department of Restorative Dentistry, Division of Biomaterials, MSC 7890, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA
^b Beijing Fine Ceramics Laboratory, Tsinghua University, Bejing 100084, China
^c University of Texas Health Science Center at San Antonio, Center for Clinical Bioengineering, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA

Received 5 March 2002;  accepted 6 August 2002.  Available online 25 September 2002.

1. Abstract

ZrO₂ (4%CeO₂) and ZrO₂ (3%Y₂O₃) coatings were deposited on titanium (Ti) and CoCrMo implants using plasma spraying and the adhesive, morphological and structural properties of the plasma-sprayed coatings were evaluated. Characterization of these coatings was performed using X-ray diffraction (XRD), scanning electron microscopy (SEM), surface roughness, hardness, and adhesive strength. XRD patterns showed that both the coatings appeared to be primitive tetragonal phase. SEM observations showed that both the ZrO₂ coatings appeared to be rough, porous and melted. The cross-section surface morphology of the coatings, coating-substrate interfaces and substrates without acid etching was very dense and smooth. After acid etching, as compared to the dense ZrO₂ coating-CoCrMo substrate interfaces, the thin gaps appeared within the ZrO₂ coating-Ti substrate interfaces. It is suggested that plasma spraying probably formed an amorphous Ti layer in the coating-Ti substrate interface that can be removed by acid etching. The average surface roughness of ZrO₂ (3%Y₂O₃) and ZrO₂ (4%CeO₂) coatings was correlated to the starting powder size and substrates. No significant difference between the hardness of all coatings and substrates was observed. The adhesive strengths of ZrO₂ (4%CeO₂) coating to Ti and CoCrMo substrates were higher than 68 MPa and significantly greater than that of ZrO₂ (3%Y₂O₃) coatings.

Author Keywords: Zirconia; Coatings; Titanium; CoCrMo alloy; Plasma spraying

2. Article Outline

1. Introduction

2. Materials and methods

2.1. Materials characterization

3. Results

4. Discussion

5. Conclusions

Acknowledgements

References

3. 1. Introduction

Zirconia ceramics were evaluated as biomaterials in the late 60s. They are currently utilized for orthopedic applications as ball heads in artificial hip joints [1, 2, 3, 4 and 5]. To date, more than 300,000 tetragonal zirconia polycrystal (TZP) ball heads have been implanted [6 and 7]. The minimal requirements for using zirconia ceramic implants are described in the ISO standard No. 13356 [8]. Clinical results have clearly indicated the benefits of using such materials, such as the prevention of a revision surgery caused by polyethylene wear debris, which is a major reason for most of the failures in the clinic [9 and 10]. Currently, TZP and alumina ceramics ball heads account for 21% of the market on artificial joints in the world. As compared to metallic ball heads, TZP ceramics possess higher wear resistance and higher corrosion resistance [11, 12 and 13]. In comparison to alumina ball heads, TZP ceramics possess higher wear resistance [14, 15 and 16], higher bending strength and fracture toughness [4, 17 and 18]. The combination of mechanical properties [4, 11, 12, 13, 14, 15, 16, 17 and 18] and excellent biocompatibility [4, 19, 20, 21 and 22] makes TZP ceramics one of the best biomaterials for prosthetic joints.

In our previous study, ZrO₂ (4%CeO₂) ceramics have been used to prepare ball heads using chemical precipitation and hot isostatical pressure (HIP) sintering. Excellent results have been reported following a preliminary 7-year follow-up research in the clinics [23]. In order to expand its use, the possibility of using ZrO₂ (4%CeO₂) powders as a coating on titanium (Ti) and CoCrMo alloy implants using plasma spraying was evaluated in this study. Adhesive, morphological and structural properties of the plasma-sprayed coatings were measured and compared to the commercial ZrO₂ (3%Y₂O₃) powders.

4. 2. Materials and methods

Materials preparation: For this study, two different ZrO₂ coatings were deposited on Ti substrates and CoCrMo alloy substrates using plasma spraying. The Ti substrates and the CoCrMo substrates were prepared by cutting commercially pure titanium (99.3% Ti) rods and CoCrMo alloy (see Table 1) rods from the Chinese Nonferrous Metal Institute into rods of size
25×5 mm³. The samples were then sandblasted with 46# Al₂O₃ particles followed by ultrasonic cleaning in reagent-grade alcohol and acetone [24, 25, 26 and 27].

Table 1. Composition of CoCrMo alloy

Two kinds of ZrO₂ starting powders were used in this study. ZrO₂ (3%Y₂O₃) powders were obtained commercially from the Chinese Nonferrous Metal Institute. The size of ZrO₂ (3%Y₂O₃) powders was in the range of 40-100 
m. ZrO₂ (4%CeO₂) powders were synthesized by chemical precipitation in our laboratory [23]. The synthesized ZrO₂ (4%CeO₂) powder morphology is shown in Fig. 1. The average size and specific surface area of the synthesized ZrO₂ (4%CeO₂) powder were 10-20 nm and 27.8 m²/g, respectively. Before plasma spraying, the ZrO₂ (4%CeO₂) powders were cold isostatically pressed at 20 MPa, smashed, and an average size of 100-175 
m was collected using 80 mesh (175 
m) and 150 mesh (104 
m) sieve. The commercially pure Ti powder (99.3% Ti, Chinese Nonferrous Metal Institute) of size 100-150 
m was used as control.

(17K)

Fig. 1. Synthesized ZrO₂ (4%CeO₂) particle morphology using transmission electron microscopy (original magnification ×200K).

The coatings were deposited using an A-3000S plasma spraying equipment. Spraying time was used to regulate the coating thickness. The plasma spraying process parameters of different coatings are listed in Table 2.

Table 2. The plasma-spraying process parameters of different coatings

2.1. Materials characterization

X-ray diffraction (XRD): Using a Rigaku D/max-rB X-ray diffractometer (Rigaku, Texas, USA), XRD analysis was used to characterize the structure of coatings. Triplicate coatings were analyzed using Cu K
radiation at 40 kV and 120 mA. The samples were scanned from 10° to 80° 2
at a scan rate of 4° per minute.

Scanning electron microscopy/energy dispersive spectroscopy: Surface morphology and composition of as-received coatings were evaluated using an energy dispersive spectroscopy (EDS) attached to a Hitachi S-450 scanning electron microscope (SEM) (Hitachi, Tokyo, Japan). Prior to evaluation, the sample surfaces were sputter-coated with gold. The cross-section surface of coatings was prepared by polishing. The polished cross-section surfaces were etched using an etch solution (841 ml of 37-38% HCl, 89 ml of 86% H₃PO₄, 63 ml of 60% HF) at room temperature for 3 min and dried at 90°C for 15 min. The cross-section surface was coated with gold and observed using a SEM.

Surface roughness: The surface roughness of triplicate coatings was measured using a surface prolifometer (Surtronic 3, Taylor-Hobson, UK).

Rockwell A-scale hardness measurement: The hardness of triplicate coatings was measured using a HR-150A hardness meter (Laizhou Exp., Shangdong, China) performed at 60 kgf (588.4 N) using a hemisphere-conical diamond (120°) indenter.

Bond strength testing: To evaluate the bond strength of the coating, the tensile strength of the coating-metal interface was measured using a universal testing instrument (AG-10TA, Shimadzu, Japan). Triplicate samples (
25×5 mm³) were evaluated using steel testing studs (
25×35 mm³). E-7 epoxy gel (Shanghai Chemical Institute, China) was used as the adhesive agent. The samples and the epoxy-coated steel studs were clamped and heated in the furnace at 100°C for 3 h, followed by furnace cooling. Tensile testing was then performed at a cross-head speed of 1 mm/min. Differences in the bond strength were statistically compared using ANOVA and were considered statistically significant if p<0.05.

5. 3. Results

X-ray Diffraction (XRD): Fig. 2 shows the representative XRD patterns of zirconia powders and coatings. No significant differences between the ZrO₂ (4%CeO₂) and ZrO₂ (3%Y₂O₃) powders and coatings were observed. XRD analysis indicated that the powders and coatings consisted of primitive tetragonal phase (diffraction number: 17-923). Two small peaks at 28.1° and 30.6° were observed on both the powders, but were absent when the powders were coated on Ti and CoCrMo substrates. XRD analysis of the Ti and CoCr Mo alloy sandblasted substrates indicated an
hexagonal phase and a rhombohedral phase, respectively.

(19K)

Fig. 2. Typical XRD pattern of different substrates and coatings.

Surface morphology: Surface morphology of different coatings is shown in Fig. 3. The surfaces of all the coatings appeared to have similar features. The Ti coatings appeared to be melted, whereas the ZrO₂ (4%CeO₂) coating appeared to be partially melted and contain some particles. The surface of ZrO₂ (3%Y₂O₃) coating appeared to contain mostly particles.

(77K)

Fig. 3. Surface morphology of different coatings. (a) ×250, Ti coating; (b) ×2.5K, Ti coating; (c) ZrO₂ (3%Y₂O₃) coating, ×250; (d) ZrO₂ (3%Y₂O₃) coating, ×2.5K; (e) ZrO₂ (4%CeO₂) coating, ×250; (f) ZrO₂ (4%CeO₂) coating, ×2.5K (original magnification ×).

At high magnification, the surfaces of all the coatings appeared to be melted. The Ti coatings appeared to be completely melted and both the ZrO₂ coatings appeared to be partially melted. Small particle aggregation, surrounded by a melted layer, and micro-cracks were observed on all coatings.

Fig. 4 shows the representative cross-section surface morphology of different coatings prior to acid etching. The cross-section surface morphology showed that all the coatings, coating-substrate interfaces and substrates appeared to be very dense and smooth. No significant differences between the coatings on Ti substrates and CoCrMo substrates were observed.

(28K)

Fig. 4. Representative cross-section surface morphology of coating on Ti and CoCrMo substrates without acid etching. C=Coating; Ti S=Ti substrate; Co S=CoCrMo substrates (original magnification ×500).

Fig. 5 shows the coating-substrates interfaces after acid etching. It was observed that the cross-section surfaces of Ti substrates became roughened after acid etching, whereas the cross-section surfaces of CoCrMo substrates were smooth. In addition, Figs. 5a and b showed that the Ti coatings on the Ti and CoCrMo substrates consisted of layers. Small gaps were observed at the Ti coating-metal substrate interface. Figs. 5c-f indicated that the ZrO₂ (3%Y₂O₃) and ZrO₂ (4%CeO₂) coatings were very dense. After acid etching, thin gaps between the ZrO₂ (3%Y₂O₃)-Ti substrate interface and ZrO₂ (4%CeO₂) coatings-Ti substrate interfaces were observed. However, the coatings on CoCrMo substrates remained dense, while no gaps were observed at the ZrO₂ (3%Y₂O₃)-CoCrMo substrate interface and ZrO₂ (4%CeO₂) coating-CoCrMo substrate interface.

(88K)

Fig. 5. Cross-section surface morphology of different coatings. (a) Ti coating, ×300, Ti S=Ti substrate, Ti C=Ti coating; (b) Ti coating, ×500, Co S=CoCrMo substrate, Ti C=Ti coating; (c) ZrO₂ (3%Y₂O₃) coating, ×640, Ti S=Ti substrate, ZY C=ZrO₂ (3%Y₂O₃) coating; (d) ZrO₂ (3%Y₂O₃) coating, X300, Co S=CoCrMo substrate, ZY C=ZrO₂ (3%Y₂O₃) coating; (e) ZrO₂ (4%CeO₂) coating, ×500, Ti S=Ti substrate, ZC C=ZrO₂ (4%CeO₂) coating; (f) ZrO₂ (4%CeO₂) coating, ×400, Co S=CoCrMo substrate, ZC C=ZrO₂ (4%CeO₂) coating (original magnification ×).

Fig. 6 shows the interfacial zone of the ZrO₂ coatings on CoCrMo substrates at high magnification. At higher magnification, no discontinuity was observed in the ZrO₂ (4%CeO₂) coating-CoCrMo substrate interface. The interfacial zone of ZrO₂ (3%Y₂O₃) coatings on CoCrMo substrates appeared to be wider than the ZrO₂ (4%CeO₂) coatings on CoCrMo substrates.

(27K)

Fig. 6. Cross-section surface morphology of different coatings. (a) ZrO₂ (3%Y₂O₃) coating, ×1.5K, Co S=CoCrMo substrate, ZY C=ZrO₂ (3%Y₂O₃) coating; (b) ZrO₂ (4%CeO₂) coating, Co S=CoCrMo substrate, ×4K, ZC C=ZrO₂ (4%CeO₂) coating (original magnification ×).

Surface roughness: Table 3 shows the surface roughness of sandblasted substrates and different coatings. The porous Ti coating exhibited the highest surface roughness, followed by ZrO₂ (4%CeO₂) coating and ZrO₂ (3%Y₂O₃) coating. The surface roughness of both ZrO₂ coatings on Ti and CoCrMo substrates was observed to be significantly increased when compared to the surface roughness of Ti and CoCrMo substrates. However, no significant difference in surface roughness was observed between the ZrO₂ (4%CeO₂) coatings on Ti substrates and the ZrO₂ (4%CeO₂) coatings on CoCrMo substrates. Similarly, no significant difference in surface roughness was observed between the ZrO₂ (3%Y₂O₃) coatings on Ti substrates and the ZrO₂ (3%Y₂O₃) coatings on CoCrMo substrates.

Table 3. Surface roughness of coatings

Hardness: Table 4 shows the Rockwell A-Scale hardness of different coatings. No significant difference in the hardness of all coatings and the substrates was observed. In addition, the hardness of ZrO₂ (4%CeO₂) and ZrO₂ (3%Y₂O₃) sintered ceramics was measured to be 92 and 80 HRA, respectively.

Table 4. Hardness of coatings

Adhesive strength: The adhesive strength of different coatings to Ti and CoCrMo substrates is shown in Table 5. The average adhesive strength of ZrO₂ (4%CeO₂) coatings to Ti and CoCrMo substrates was greater than 68 and 67.7 MPa, respectively, with fracture occurring within the adhesive agent. The adhesive strength of ZrO₂ (3%Y₂O₃) coatings to Ti and CoCrMo substrates were 32.3±0.8 and 24.7±2.6 MPa, respectively. It was observed that the adhesive strength of ZrO₂ (4%CeO₂) coating to the Ti and CoCrMo substrates was significantly greater than the ZrO₂ (3%Y₂O₃) coatings to the Ti and CoCrMo substrates. The adhesive strength of ZrO₂ (4%CeO₂) coating to the Ti and CoCrMo was also observed to be significantly greater than the Ti coatings to Ti and CoCrMo substrates. It was worth noting that the ZrO₂ (3%Y₂O₃) coatings were completely peeled off during the adhesive strength tests, but the Ti coatings were destroyed and parts were peeled off.

Table 5. Adhesive strength of different coatings to Ti substrates

6. 4. Discussion

Numerous problems with the plasma-sprayed coatings have also been cited, including variation in bond strength between the coatings and the metallic substrates, nonuniformity in coating density as a result of the process, poor adhesion between the coatings and metallic substrates, and micro-cracks on the coating surface [28, 29, 30 and 31]. In this study, the adhesive, morphological and structural properties of the plasma-sprayed coatings were measured and compared to the commercially ZrO₂ (3%Y₂O₃) powders.

Zirconia is a well-known polymorph that occurs in three forms: monoclinic (M), cubic (C) and tetragonal (T). Pure zirconia is monoclinic at room temperature [1 and 32]. This phase is stable up to 1170°C. Above this temperature it transforms into tetragonal and then into cubic phase at 2370°C. During cooling, a T-M transformation takes place in a temperature range of about 100°C below 1070°C while cooling is associated with a volume expansion of approximately 3-4%. Stresses generated by the expansion originate cracks in pure zirconia ceramics that, after sintering in the range 1500-1700°C, break into pieces at room temperature. The addition of `stabilizing' oxides, like CaO, MgO, CeO₂, Y₂O₃, to pure zirconia allows the generation of multiphase materials known as partially stabilized zirconia (PSZ) whose microstructure at room temperature generally consists of cubic zirconia as the major phase, with monoclinic and tetragonal zirconia precipitates as the minor phase [33]. PSZ can also be obtained in the ZrO₂-Y₂O₃ system. However, in this system, it is also possible to obtain ceramics formed at room temperature with a tetragonal phase only, called TZP [34 and 35]. TZP materials, containing approximately 2-3% mol Y₂O₃, are completely constituted by tetragonal grains with sizes of the order of hundreds of nanometers. The fraction of T-phase retained at room temperature is dependent on the size of grains, on the yttria content, on the grade of constraint exerted on them by the matrix. In our experiments, XRD analysis indicated that the powders and coatings consisted of primitive tetragonal phase (diffraction number: 17-923), with no cubic or monoclinic phases observed. No significant differences between the ZrO₂ (4%CeO₂) and ZrO₂ (3%Y₂O₃) powders and coatings were observed. Two small peaks at 28.1° and 30.6° were observed on both the powders, but were absent when the powders were coated on Ti and CoCrMo substrates suggesting structural alterations as a result of high temperature during plasma spraying [36 and 37].

It was also observed from this study that the coating roughness was correlated to the size of the starting powders and substrates. Since the size of Ti and ZrO₂ (4%CeO₂) powders was significantly larger than ZrO₂ (3%Y₂O₃), the ZrO₂ (3%Y₂O₃) coatings were observed to have the lowest surface roughness. In comparison to Ti powders with similar size, the ZrO₂ (4%CeO₂) powders were in aggression due to cold isostatical pressure. As such, the Ti coatings were observed to have a significantly higher surface roughness compared to ZrO₂ (4%CeO₂) coatings. Similarly, since the CoCrMo substrates had higher surface roughness than Ti substrates, the same coatings on CoCrMo substrates were observed to have higher surface roughness compared to the coatings on Ti substrates.

The coating surface roughness of different coatings was in agreement with observations using SEM. At low magnification, the surfaces of all the coatings were observed to be rough and porous, with some particle aggregation zone and some melted zone. At high magnification, SEM micrographs also indicated small differences in surface morphology between the Ti- and ZrO₂-coated surfaces. At high magnification, the Ti coatings appeared to be completely melted and both the ZrO₂ coatings appeared to be partially melted. Small particle aggregation, surrounded by a melted layer, and microcracks were observed on all coatings. Microcracks on all the ZrO₂ surfaces were also observed at high magnification. The presence of microcracks on all plasma-sprayed surfaces was in agreement with other studies [38].

Prior to acid etching, the cross-section surface morphology showed that all the coatings, coating-substrate interfaces and substrates appeared to be very dense and smooth. After acid etching, the cross-section surfaces of Ti substrates indicated roughened surfaces, whereas the cross-section surfaces of CoCrMo substrates were smooth. In addition, the Ti coatings on the Ti and CoCrMo substrates consisted of layers, with small gaps observed at the Ti coating-metal substrate interface. Thin gaps between the ZrO₂ (3%Y₂O₃)-Ti substrate interface and ZrO₂ (4%CeO₂) coatings-Ti substrate interfaces were also observed. However, the coatings on CoCrMo substrates remained dense, with no gaps observed at the ZrO₂ (3%Y₂O₃)-CoCrMo substrate interface and ZrO₂ (4%CeO₂) coating-CoCrMo substrate interface. It is known that the plasma-spraying process utilizes a gas stream to carry powders, which are then passed through electrical plasma produced by a low voltage, high current electrical discharge [39 and 40]. As the sprayed semi-molten powders solidify, an amorphous layer of amorphous Ti was formed within the coating-Ti substrate interfaces. As such, it was suggested that the gaps were likely due to the removal of amorphous Ti layer by acid etching.

The average adhesive strength of ZrO₂ (4%CeO₂) coatings to Ti and CoCrMo substrates was greater than 68 and 67.7 MPa, respectively, with a fracture occurring within the adhesive agent. It was observed that the adhesive strength of ZrO₂ (4%CeO₂) coating to the Ti and CoCrMo substrates was significantly greater than the ZrO₂ (3%Y₂O₃) coatings to the Ti and CoCrMo substrates. The adhesive strength of ZrO₂ (4%CeO₂) coating to the Ti and CoCrMo was also observed to be significantly greater than the Ti coatings to Ti and CoCrMo substrates. It is known that the mechanical properties of TZP ceramics were dependent on the metastable nature of the tetragonal grains. A critical grain size exists, linked to the yttria concentration, above which spontaneous T-M transformation of grains takes place [37]. Inhibition of the T-M transformation was reported in fine-grained structure [37]. As such, the adhesive strength differences between different ZrO₂ coatings to substrates were suggested to be attributed to the different tetragonal grain size within ZrO₂ (4%CeO₂) and ZrO₂ (3%Y₂O₃) coatings.

It was also observed in this study that the thickness of ZrO₂ (4%CeO₂) coatings was significantly lesser than the ZrO₂ (3%Y₂O₃) coatings, suggesting a higher stress existing within thinner ZrO₂ (4%CeO₂) coatings as compared to ZrO₂ (3%Y₂O₃) coatings, and thus thereby attributing to the differences in adhesive strength observed. An interesting characteristic of the T-M transformation was the formation of compressive layers on the zirconia surface [41]. Surface tetragonal grains were observed to be unconstrained by the matrix, and could be spontaneously transformed to monoclinic. Such spontaneous transformation was reported to induce compressive stresses at a depth of several microns under the surface [41]. The surface phase transition and the consequent surface hardening was thus suggested to have a relevant role in improving the mechanical and wear properties of zirconia parts, with the thickness of the transformed layer being one of the limit conditions. However, more studies will be needed to provide a better understanding of the adhesion mechanism of ZrO₂ and Ti coatings to the Ti and CoCrMo substrate. In addition, in vitro and in vivo biocompatibility studies of ZrO₂ coatings are ongoing.

7. 5. Conclusions

In this study, the adhesive, morphological and structural properties of plasma-sprayed ZrO₂ (3%Y₂O₃) coatings, ZrO₂ (4%CeO₂) coatings, and titanium (Ti) coatings on Ti and CoCrMo surfaces were evaluated. It was concluded from this study that coatings using synthesized ZrO₂ (4%CeO₂) and commercially available (3%Y₂O₃) powders exhibited similar morphological and structural properties. In addition, it was concluded that implants coated using synthesized ZrO₂ (4%CeO₂) powders resulted in higher adhesive strength as compared to the use of commercially available (3%Y₂O₃) powders for coatings.

8. Acknowledgements

This study was funded by the National Institute of Health (1RO1AR46581) and Chinese 973 Hi-Tech Project (G19990647-02). The authors also thank Dr. Xinjian Deng and Dr. Chaojun Zheng for their help in plasma spraying.

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Corresponding author. Department of Restorative Dentistry, Division of Biomaterials, MSC 7890, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, , San Antonio, TX 78229-3900, , USA. Tel.: [phone]; fax: [phone]; email: [e-mail]

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