Andy Sturgeon joined TWI at the end of 1990 as a Principal Research Engineer in the Arcs, Lasers and Sheet Processes Department. After obtaining a degree in physics, he began his research career at the Centre for Advanced Materials Technology at Warwick University, he gained a doctorate for his work on bonding mechanisms between glasses and ceramics to metals. He spent two years with the ceramics group at Rolls-Royce plc, involved with projects introducing advanced ceramic components into aero-engines. This was followed by a further two years with Alcan International Ltd, developing new and novel processing routes to ceramic and ceramic composite materials, in particular sol-gel and chemically bonded ceramics (for example high strength cements and reaction formed ceramics).
Andy chairs TWI's Surface Engineering Team. He is currently investigating HVOF thermal spraying as a process for putting down high quality coatings for wear, corrosion and electronic applications.
Orthopaedic implants, in particular hip joint replacement, represent an important area in the health care industry - currently in excess of 20,000 operations annually. Andy Sturgeon reviews the latest technology.
The present approach to hip joint replacement is based on that developed by Charnley in 1962. A metallic stem of stainless steel, Co-Cr alloy, or more recently titanium, inserted into the femur bone is located in an ultra high molecular weight polyethylene (UHMWPE) socket. The polyethylene cup and metallic stem are both secured in position using polymethylmethacrylate (PMMA) cement.
Although very successful, the lives of current implants rarely exceed 12 years before a repeat operation is needed, due to implant loosening resulting from bone reabsorption around the cemented implant.
There is a trend to move away from traditional metals secured with PMMA cement, to implants whose surfaces are coated with materials which promote direct bonding with the surrounding bone tissue. A good example is hydroxyapatite, a form of calcium phosphate [Ca 10 (PO 4 ) 6 (OH) 2 ], which has a similar composition to the mineral phase found in bone.
These materials, described as being bio-active, mimic natural tissue and promote bone growth on to, rather than bone reabsorption around the implant. Coatings of hydroxyapatite are now being deposited on to the stems of orthopaedic implants to achieve cementless fixation to bone. Such coatings are believed to reduce implant susceptibility to loosening.
Many processes have been investigated for depositing hydroxyapatite coatings, including:
- electrophoretic deposition
- physical vapour deposition
- glass enamelling
- Sol-gel
- air plasma spraying
Air plasma sprayed coatings are the most successful to date and have gained clinical acceptance.
The current consensus is that the coating should contain only hydroxyapatite as the primary crystalline phase. An appreciable presence of other forms of calcium phosphate such as tri-calcium phosphate or an amorphous phase are regarded as undesirable, and are known to result in rapid loss of the coating once implanted.
With the plasma spray process it is difficult to achieve reproducible control over retention of the hydroxyapatite phase and level of crystallinity in the coating. This is in part due to the high temperature of the plasma environment (typically >5000°C) which can lead to decomposition and excessive melting of the hydroxyapatite powder during spraying.
Recent investigations
Recent work at TWI has investigated the preparation of hydroxyapatite coatings using the high velocity oxyfuel (HVOF) thermal spray process, Fig.1. HVOF offers a thermal spraying process with a less severe thermal environment than plasma spraying and an increase in powder impact velocities.
It is a process more suited to the spraying of thermally sensitive powders such as hydroxyapatite, and should give better control of coating structure. Process temperatures are lower at typically 2800-3200°C and particle velocities higher at about 600-800 m/sec compared to between 200-400 m/sec for air plasma spraying.
HVOF Hydroxyapatite
An X-ray diffraction spectrum collected from a powder used to prepare coatings of hydroxyapatite is given in Fig.2. This shows that the powder was highly crystalline with no amorphous content. The peak positions and relative heights correspond to those for crystalline hydroxyapatite (JCPDS file 9-432). There are no peaks corresponding to other calcium phosphate phases such as tri-calcium phosphate. The powder was of a nominal particle size range 25-45µm and prepared using a precipitation route. Each individual powder consists of an agglomeration of fine particles.
Coatings of hydroxyapatite on titanium were prepared using the Top Gun HVOF system with ether acetylene or hydrogen fuels.
X-ray diffraction spectra for the two coatings are given in Fig.3 and 4. The observed peak positions reveal that hydroxyapatite is the only crystalline phase present in both coatings. There is, however, a marked difference in the degree of crystallinity. The presence of an amorphous or micro-crystalline content is indicated by the appearance of a hump in the X-ray spectrum in the 2 Θ range of 30-50°. A large hump area relative to the area under the sharp peaks represents a higher amorphous content. The X-ray diffraction spectra show that the coating prepared using hydrogen as the fuel has a noticeable amorphous content. Acetylene fuel gives a coating with a very low amorphous content.
Discussion
The HVOF thermal spraying process can be seen to deposit coatings which retain hydroxyapatite as the only crystalline phase present. The X-ray diffraction results clearly demonstrate that the parameter settings and fuel gas used will influence the level of crystallinity. The acetylene fuel conditions give a coating with good retention of the hydroxyapatite phase and a high level of crystallinity. Scanning electron microscope images of fractured coating sections are shown in Fig.5 and 6.
Acetylene fuel produces a coating containing regions which are fused adjacent to regions consisting of fine particles. These observations are consistent with partial melting of an initial powder comprised of agglomerated particles, with the outer region becoming molten and the inner region remaining as un-melted fine particles. On impact these unmelted particles produce regions of fine particles in the coating.
The use of hydrogen fuel gives a different coating morphology. A higher level of melting appears to have occurred, with the fracture surface showing an absence of unmelted regions within the coating.
The temperature reached by the powder must in part depend on the temperature and heat content of the surrounding combustion environment. Values for the exhaust gas temperature and heat content have been estimated for the spraying conditions used in this work, see Table. Also given in the Table are HVOF particle velocities recently measured at TWI for alumina powder with a 15µm mean size.
Combustion characteristics
| Characteristic | Acetylene
fuel gas | Hydrogen
fuel gas |
| Oxyfuel ratio | 1.63 | 0.35 |
| Flame temperature, °C | 3160 | 2835 |
| Combustion power, kW | 68 | 120 |
| Mean particle velocity, m/sec | 420 | 740 |
The use of hydrogen as the fuel gas can be expected to give a slightly lower combustion temperature but a considerably higher heat content and higher particle velocity compared to acetylene fuel. The higher level of melting observed for the coating prepared using hydrogen fuel can in part be attributed to the higher heat content of the combustion environment together with a higher speed and associated kinetic energy of the powder prior to impact. Any molten material formed during the spraying process will be rapidly solidified on impact at cooling rates quoted at 10 6 to 10 8° C/sec. Such rapid rates of cooling will often lead to the formation of amorphous or microcrystalline material.
As was discussed earlier, the presence of amorphous material in coatings of hydroxyapatite is often considered undesirable. The results presented here would suggest that the spraying conditions used with acetylene fuel are more suitable.
Conclusion
These initial results are encouraging and demonstrate that good quality hydroxyapatite coatings can be prepared using the HVOF process. Further work is now underway at TWI to determine the role fuel gas type has on coating adhesion, and to establish the extent to which coating morphology can be varied in a controlled manner whilst still retaining the high level of crystallinity.
Acknowledgements
The author would like to express thanks to Dr J C Knowles of the Interdisciplinary Research Centre in Biomedical Materials, Queen Mary and Westfield College for assistance with the X-ray diffraction and particle size analysis.
