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Addressing processing problems associated with plasma spraying of hydroxyapatite coatings
Biomaterials
17 (1996)
537-544
published by Else&x Science Limited Printed in Great Britain. All rights reserved
0
1996
ELSEVIER
0142-9612/96/$15.00
Addressing processing problems associated with plasma spraying of hydroxyapatite coatings * P. Chean& and K.A. KhorS t School of Applied Science and t School of Mechanical and Production University, Nanyang Avenue, Singapore 2263, Singapore Biomedical
coatings
linity (for positive necessary Thermal
generally
biological
to enhance
have to satisfy specific
responses),
spray processes
such as hydroxyapatite
implants
have been widely
generated
concerns
investigations on coating performance
acknowledged,
using HA coatings
such as bioresorption, such as powder
degradation
morphology
In addition,
conditions
tive or secondary
treatment
problems
associated
feedstock
morphology
efficiency
and produce
Keywords:
certain
opposing
via thermal
stages
with plasma
to attain the desired spray coating
suitable
an acceptable
coating
structure.
plasma
sprayed
spraying,
composition
Variation
has Recent
has significant
influence and
in process
and mechanical
inconsistencies
In order to reach some acceptable in commercially
results.
available
the means to achieve
creating
This paper
powder
highlights
that tailoring processes
properties,
level
HA the necessary
the need to introduce
conditioning
microstructure,
HA coatings.
variability
of HA and suggests
through
in coated
poor performances
and chemical
constrain
alone; therefore,
and properties
Hydroxyapatite,
related
variability
factors severely
spraying
are
fixation.
of HA materials
of several
and bone apposition.
to control existing
it may be necessary
and improve
active biomedical
can induce microstructural
of reliability,
of crystal-
These
functionally
structure
of the coating.
porosity.
The benefits
of thermally
have shown that process
such as phase composition,
performance
coating
implants.
and reliability
such as a high degree healing
used to deposit
but the occurrence
Nanyang Technological
and optimal
post-operative
that have an effect on the service feedstock.
adhesion
(HA), onto prosthetic
over the consistency
characteristics
parameters
accelerate
have been frequently
coatings,
requirements
good coating
biocompatibility,
Engineering,
other innova-
some of the
the powder
can aid the deposition
bioactive
phases
Received 9 March 1995; accepted 15 May 1995
adverse reaction by the provision of a biocompatible phasel. Some important coating characteristics that are generally expected of biomedical coatings are good biocompatibility of the desired phase and crystallinity, good coating integrity and adequate porosity. Phase and crystallinity are important to ensure that the appropriate biological responses occur. The formation of transitional phases can elicit unfavourable resorption and toxicity. Coating integrity is needed to ensure that necessary mechanical characteristics are present to keep the coating intact. Porosity of the optimum size can improve fixation by promoting bone ingrowth to create mechanical interlocking’. A bioceramic commonly used as coating material for prosthetic implants is HA. It is a bioactive material with chemical composition similar to natural bone. It has been known to form strong biological bonds with bony tissue without the presence of soft fibrous tissues3’4. The provision of a high calcium and phosphorus-rich environment promotes rapid bone formation within the vicinity of the implant. HA also
Coatings used for biomedical applications are often subjected to many stringent constraints. In general, materials that are applied onto metallic prostheses often adopt a secondary role in enhancing the characteristics of the implants. As coatings, they are not intended to substitute existing materials but more correctly to improve the characteristics of a fully functional implant’. Good prosthetic designs normally have provisions to accommodate eventualities such as coating failure without jeopardizing the function of the implant. This perception is important to ensure that the role and purpose of coatings are not misunderstood. Coatings have specific functions ranging from improving fixation by establishing strong interfacial bonds, shielding the metallic implant from environmental attack or leaching effects, promoting fast tissue growth and interaction by the presence of a catalyst material, hydroxyapatite (HA), and minimizing *Presented in part at the 8th International Conference on Biomedical Engineering, 7-10 December 1994, Singapore. Correspondence to Dr K.A. Khor. 537
Biomaterials 1996, Vol. 17 No. 5
538
Problems
associated
establishes strong interfacial bonds with titanium implants. Its excellent biointegration makes HA an ideal choice for use in orthopaedic and dental applications. HA coatings can be applied in several ways4-g. The use of thermal spraying techniques to coat HA onto implants is popular due to certain advantages such as simplicity, high deposition rates, low substrate temperature, variable coating porosity, variable phase and structure. With this process, any material that does not decompose or vaporize during melting is suitable for thermal spraying’“. Particulate materials are injected into the high temperature heat sources where they are melted and propelled at high velocity towards a substrate. The molten droplets flatten on impact as splats to form a lamellar coating structure. Although thermal spraying is conceptually a simple process, it is, however, complex in operation. Some 100 variables are associated with the spraying process which have interrelated influence on the microstructure and properties of the final coatinglo. Among these are the operating conditions of the plasma spray system (power setting, choice of auxiliary gas, feed rate, etc.), the size of the plasma jet hot zone and the heat and momentum transfer from the plasma jet to the particlesl’. The turbulent profile of the plasma jet affects the quantity of air entrainment and, hence, the heat transfer rates to particles travelling through the plasma process zone. While it is easy to form any coating, it is difficult to produce a coating with the desired coating characteristic. Furthermore, there is generally a concern over the consistency and reliability of the coating quality. This is particularly true for HA coatingsl’. Recent studies have concentrated on the selection of spraying parameters that affect the coating microstructure and properties, hence coating performance. The influence of the morphology and denseness of the feedstock was also investigated13. A problem pertinent to the plasma-sprayed HA coating is the generation of an amorphous phase, along with other non-bioactive calcium phosphate phases following plasma spraying14. The formation of the amorphous phase is apparently associated with partial dehydroxylation of the HA during the plasma spray process15. The presence of an amorphous calcium phosphate phase is undesirable because natural bone HA is crystalline, thus the integrity of the bone-implant interface is compromised16. In addition, the strong resorption of the amorphous phase may cause mechanical and adhesive instability in the coating. Degradation of HA coatings with high amorphous phase content occurs by d&-adhesion of cracked lamellae and dissolution of the remaining lamellae during in vitro tests17. Mechanical tests show that failure of the bone-coating-implant interface consistently occurred within the bone for amorphous and poorly crystallized HA coatings as a result of bioresorptionl’. Some factors identified as influencing the coating stability in plasma-sprayed HA coatings during in vivo experiments include the particle size distribution, degree of melting (determined by the feed position of the powders into the plasma flame) and post-spray treatmentlg* “. As results from recent studies have indicated, thermal spraying of HA coating is indeed a formidable challenge to a materials Biomaterials 1996, Vol. 17 No. 5
with
plasma
spraying
of HA coatings:
P. Cheang
and K.A.
Khor
processor due to the low degree of ‘forgiveness’ of both the process and the material concerned. The plasma spray process demands a series of conditions such as appropriate particle size range, power setting, particle morphology, etc., for optimal coatings, while the material concerned, HA, reacts strongly to rapid solidification following the plasma spray and yields a series of amorphous and metastable phases. In this study, the problems encountered during the thermal spraying of HA-based materials are examined. Particular emphasis is placed on the characteristics of the powder feedstock and their influence on the microstructural and mechanical properties of plasmasprayed HA-based coatings.
EXPERIMENTAL TECHNIQUE The HA feedstock was prepared by wet reacting orthophosphoric acid with calcium hydroxide in specific mole concentratior?. The fine precipitates were later used to produce three other powders: calcined HA (CHA), spray-dried HA (SDHA) and spheroidized HA (SHA). CHA was produced by oven drying at -180°C and calcining the feedstock at 800°C for 4 h. SDHA was prepared by blending HA with 2% organic binder and spray dried using a two-fluid nozzle (Lab-Plant SD-04 system, UK). The size range of the SDHA is approximately 2-20 pm. SHA was produced in-house by flame spraying the 53-75 pm CHA faction using a combustion flame gun PF-73 (Miller Thermal Inc., USA) into distilled water. The HA coatings were produced via plasma spraying using a 40 kW DC plasma torch (SG-100 Miller Thermal Inc., USA) and assisted by a computerized closed loop rotor-feed hopper. Typical spraying conditions are tabulated in Table 1. Optical light (Nikon Metallurgical System) and scanning electron microscopy (Cambridge S360 SEM) were used to evaluate the surface morphology and sectioned microstructure of coatings and powders. A powder X-ray diffractometer system (Philips MPD 1880) was used to determine the phase composition of the coatings.
Table
1
Plasma spraying
Main arc gas (Ar) Auxiliary gas (He) Arc current Arc voltage Spraying distance
’ 1 psi
parameters 50 psi” 15-50 psi 800 A 32-35 V 120 mm
= 6.9 kPa.
RESULTS Particle morphology Calcined HA (CHA) The CHA powders are essentially angular in shape with a porous structure comprising fine agglomerated particles with size 2-5 pm. Particle size between 53 and 75 pm were used for plasma spraying (Figure 2).
Problems
associated
with plasma spraying
of HA coatings: P. Cheang and K.A. Khor
Percent~gc I%
539
Plasma Vs Flame Spheroidization of HA
__
30
20
10
0 10
20
32
-
Flame
m
Figure 4 Particle size distribution spheroidized hydroxyapatite. Figure 1
Calcined hydroxyapatite
53 Particle
45
siu
76 /microns
Plasma
of plasma
and flame
powders.
Spray dried HA (SDHA) The spray-dried powder sizes range between 2 and 20 pm and is similar in structure to CHA except for its spherical geometry (Figure 2).
Spheroidized HA (SHA) These are highly dense spherical particles with a glassy smooth surface texture. Particle sizes range between 20 and 53 pm (Figure 3). Comparison
a Figure 2
Spray-dried
hydroxyapatite
powders.
b Flgure 3
Flame spheroidized
hydroxyapatite
powders.
Figure 5 a, As-sprayed surface of calcined hydroxyapatite coating; b, cross-section of calcined hydroxyapatite coating. Biomaterials 1996, Vol. 17 No. 5
Problems
540
associated
with
plasma
spraying
of HA coatings:
P. Cheang
and K.A.
Khor
led to deposition and phase inconsistency caused by the impact of irregularly sized particles. Completely different surface and cross-sectional microstructures were observed using SHA powders (Figure 7). The regular formation of neatly stacked disc-like splats produced a flat and smooth surface profile. The cross-sectional view of the coating microstructure reveals a lamellar structure that is free of large macrovoids. The presence of good interlamellar contact and the absence of macropores and unmelted particles greatly improves coating integrity.
X-ray diffkaction (XRD) analysis XRD analysis of the plasma-sprayed CHA coatings revealed the presence of amorphous calcium phosphate, tetracalcium phosphate (TET), tricalcium phosphate (TCP) and calcium oxide (CaO) in addition to crystalline HA. They differed considerably from the starting material that was essentially pure and crystalline HA. Comparing the phase content of the SDHA coatings, it was observed that both the crystallinity of
b Figure 6 a, As-sprayed apatite; b, cross-section coating.
surface of spray-dried hydroxyof spray-dried hydroxyapatite
between plasma spheroidized and combustion flame SHA powders have shown that the latter batch possessed a more uniform distribution (Figure 4) and consistent chemical composition among the particles (determined using energy dispersive X-ray analysis)“.
a
Coating microstructure Using angular CHA powders produced relatively poor coating microstructures. They consisted of micro- and macro-pores, irregular splats and unmelted particles with limited interlayer adhesion (Figure 5). Considerable amounts of unmelted particles were present which contributed to the formation of large pores and cavities. These coatings have poor structural integrity because of limited interparticle cohesion. Some microstructural improvements were observed using SDHA powders. The reduction in the overall porosity was attributed to better flow behaviour of the spherical powder (Figure 6). However, the weakly agglomerated structure of both powders did not eliminate the problem of particle break down which Biomaterials
1996, Vol. 17 No. 5
b Figure 7 a, As-sprayed surface of spheroidized hydroxyapatite coating; b, cross-section of spheroidized hydroxyapatite coating.
Problems
associated
with
plasma
spraying
of HA coatings:
Calcined
HA
(53-75
JU,I)
(45-53
Spray
Z-Theta
Figure 8 X-ray diffraction spray-dried hydroxyapatite tite coatings.
Dried
HA
(5-20
P. Cheang
pm)
pm)
Angle
of calcined hydroxyapatite, and spheroidized hydroxyapa-
HA and peak intensity of CaO were lower (Figure 8). For SHA, the peak intensity of HA was similar to CHA, although some degree of line broadening was observed. The level of CaO was found to be the lowest among the three powders.
DISCUSSION Thermal spraying processes involve complex interactions between the heat source, particulates and substrate. Particles injected into the plasma zone experience extreme heating rates and temperatures exceeding 10 000”Cz3. As a result, the majority of the powder, within a critical size range, melts to yield an aerosol of molten droplets. Large particles, above the critical size range, may experience only partial melting while the small particles may have a layer of vaporized material surrounding the liquid droplets. Hence, conditions arising from plasma-particle interactions such as short residence time (milliseconds), high gas velocities and inconsistent trajectory path of different particle sizes generally cause incomplete and nonThese heated uniform treatment of particulates. particles further interact with the surrounding atmosphere and subsequently impact at high velocity and cooling rate (about 10” KS-‘) to form a coating. The solidification process of the molten droplet is obviously non-equilibrium. These conditions promote the formation of metastable phases and a complex microstructure comprising macro- and micro-porosity, fused and partially molten particles that have limited
and K.A. Khor
541
interparticle cohesion. In general, the deposition profile of the as-sprayed coating depends largely on the operating state of the plasma and the physical characteristics of the powder. These ultimately introduce large variability that may or may not be desirable, depending on the application concerned. However, proper understanding and selection of essential parameters could minimize unfavourable properties. Investigations into the thermal spraying of HA coatings have shown that variation in operating parameters can cause significant changes to the microstructural and mechanical properties of the coating. Even though similar starting materials were used, process-related factors have significant effects on the final coating characteristic. XRD analysis of both powder and coating reveal distinctly different peak profiles. Variation in the powder characteristic in terms of morphology, structure and size alters the coating characteristic in terms of integrity, microstructure, surface profile and degree of crystallinity. This therefore suggests that in order to achieve coating consistency, hence similar mechanical and biological behaviour, similar powder type and configuration must be used, provided all other parameters remain constant. Achieving this in practice is difficult because of the wide variability of commercially available powdersZ4. Coating generated from agglomerated spray-dried powder has differing characteristics from that of densely fused powders. The variation in the coating microstructure is caused by differing stability of the powder, flow characteristic, melting behaviour, deposition rate and droplet size. The stability of the feedstock affects the flow behaviour, deposition consistency, phase content and degree of crystallinity in the as-sprayed coating. The flow characteristic of the powder is dependent on factors such as particle size and size distribution. Finer particles have greater difficulty to flow consistently due to closer particle spacing and static effects. This would lead to intermittent feeding, inconsistent deposition and coating unevenness. Powders with a narrow particle size range have better flow behaviour compared to mixed sizes comprising large and fine particles. Fines generated from a weakly bonded particle also disrupt flow. Under the turbulent environment of the plasma jet, these weakly agglomerated particles are likely to break down to create further complications during deposition. The physical nature of the particle has an effect on the melting characteristics. Therefore, differences in melting behaviour will occur between dense and porous particles. Porous particles have the tendency to form hollow spheres that further increase coating porosity. With dense particles, the formation of a filled spherical droplet would lead to a more uniform splat pattern. Quality coatings that are both smooth and dense are normally formed by the deposition of monosized particles that are sufficiently molten. The sizes of the impacting particles have an effect on the phase composition. The thermal state of the particle is governed by the heat absorbed during plasmaparticle interaction. Some factors influencing the Biomaterials
1996. Vol.
17
No. 5
542
Problems
associated
degree of heat absorbed are temperature, heating rate, residence time, particle conductivity, thermal geometry, melting point, heat of fusion and density. For a constant plasma environment, a smaller particle size is expected to reach higher temperature. The ease with which materials melt during thermal spraying is defined by the difficulty of melting factor (DMF). This is also related to the maximum fusible particle size (SC). This suggests that for any plasma condition and material system, there is a critical particle size that generates optimum coating quality. Deviation from this optimum value would degrade the quality of the coating by introducing excess porosity, transitional phases and amorphous content. However, the critical value for optimum biocompatibility and mechanical integrity may not necessarily be the same. Previous investigation with agglomerated HA powders has shown that the percentage of unmelted crystalline phases increases with increasing particle sizes above Scz5. Below this value the degree of amorphous nature and transitional phases increased with decreasing particle size. But a more coherent coating microstructure is formed. Thus, in order to achieve a high degree of crystallinity in the as-sprayed coating, a size slightly larger than the largest fusible particle size (SC) must be used. However, too large a size would introduce excessive porosity that could weaken the coating mechanical properties. Since the size of impacting particles severely affects both phase and crystallinity, the stability of the particle during plasma-particle interaction becomes important. To ensure that the optimal thermal states are achieved during heating, the geometry of the particles must be maintained. Investigation with weakly agglomerated HA powders has shown that particle breakdown occurs to an extent that a complex multiphase coating emerges that has varying degree of crystallinity, This inadvertently introduces undesirable properties such as poor biocompatibility and mechanical integrity. Thermal sprayed ceramic coatings are generally porous. This characteristic was initially thought to benefit biomedical application because of the possibility of mechanical fixation through bony ingrowth. Porosity inherent of thermally deposited coatings are essentially bimodal in nature. Pores exist as large cavities above 10 pm to fine pores of less than 1 pm. Fine pores are generally considered as interlamellar pores that occupy 70% of the overall porosity’“. As thermally sprayed coatings are built up of successive layers of lamellar splats, the degree of good contact between lamellae greatly affects the quality of the coating bond strength. This unique arrangement of lamellae renders the coating stiffer in the planar direction as opposed to the perpendicular direction. This anisotropic behaviour is advantageous to applications subjected to shear forces as is normally experienced in the stem section of hip implants. However, pores that normally promote bone ingrowth are generally in the ZOO-GOO pm rangeZ7. The formation of such pores is not encouraged because of the severely weakening effects on mechanical properties. Considering the requirements of a biomedical
with plasma
spraying
of HA coatings:
P. Cheang
and K.A. Khor
coating, it appears that some factors are difficult to achieve while others are conflicting in nature. Thermal spraying invariably causes physico-chemical changes to the starting material, Good biocompatibility requires that the coating material contain a highly crystalline form of HA that is bioactive. The other forms of transitional phases such as TET, TCP and amorphous calcium phosphate have differing biologiAs previously mentioned, highly cal responses. crystalline HA is achieved by using a particle size that is slightly larger than the critical melting size. With larger size, incomplete melting occurs which retains the original crystalline form of HA. On impact, semi-molten particles interlock mechanically to form a complex array of pores and irregularly shaped compacts. Because of the solid nature of the particles, and the high degree of porosity, the mechanical properties are generally poor. Having good structural integrity, on the other hand, requires a regular stream of adequately molten droplets that spread in an orderly fashion on impact, without fragmenting, to form a uniform lamellar structure with minimal porosity. Good adhesive and cohesive properties can be expected from this deposition. In fact, recent tensile adhesion test results support this sensitive dependence of mechanical properties on microstructure. The bond strengths of porous CHA coatings are lower than densely packed SHA coating?. However, well-melted coatings are generally amorphous and contain transitional phases that are more bioresorbable. Here, the mutually opposing effects of biocompatibility and mechanical integrity are clearly demonstrated. Although the long-term benefits of a permanent biocompatible coating are not known, short-term advocators of the HA coating to induce rapid bone growth and fixation emphasize the greater importance of biocompatibility over mechanical properties. Should a more permanent coating be required over extended periods then the engineering of a more coherent coating becomes crucial and obviously more difficult. The constraints introduced by opposing effects of both coating integrity and biocompatibility limits the possibility of achieving the desired coating characteristics in a single-step spraying process. Certainly the use of post-treatment processes involving thermal and laser techniques provides greater opportunity to attain the necessary requirements. Preliminary investigation of laser-treated coatings favours the heat treatment processing to induce phase transformation rather than melting for densification because of apparent crackingzg. A recent investigation using HA-based composite powders suggest a composite coating containing a bioinert ceramic and a bioactive or bioresorbable HA”‘. Such a technique would exploit the strength of the spraying process for melting and deposition and at the same time achieve its dual characteristic based on powder design. Spheroidized composite powders containing two different materials have already been produced and these coatings are currently under investigation. Using hot isostatic pressing to further enhance the coating property is certainly an avenue worth considering31.
Problems
associated
with
plasma
spraying
of HA coatings:
P. Cheang
CONCLUSIONS The problems encountered with the use of thermally sprayed HA coatings are mainly improper melting of the feedstock, reproducibility and satisfying biomedical requirements. Mixed successes with the use of HA coated implants generates a level of uncertainty that the material is unpredictable. This investigation from the purely processing perspective suggests that suitable powder conditioning can help alleviate some of the existing variability originated from a lack of standardization of operating parameters. HA feedstocks that are used for thermal spraying vary considerably from composition, phase, crystallinity, particle size, shape and structure. Most HA powders are of the agglomerated, spray-dried genre. These powders, unfortunately, have a tendency to break down in the turbulent plasma and result in a coating that is built up of droplets whose size range is a mixture of the original size range and broken down fragments. In addition, previous work on plasma spraying of HA coatings utilized powder sizes that ranged between 1 and 100 pm. This will give rise to a wide array of plasma-particle reactions. The resultant coating from this wide particle size range will not provide the level of consistency that is desired. Considering that the coating microstructure is highly dependent on powder characteristics, this alone is sufficient to cause variation. The selection of particle size without considering the critical particle size is certain to lead to the formation of unfavourable weak amorphicity and mechanically phases, microstructures. Satisfying the requirement for pore sizes in the range 200-400 pm is difficult with thermal spraying. Additional post-treatment processes Both biocompatibility and coating are necessary. integrity are competing factors that are mutually exclusive. For short term application of HA coating where the coating exists temporarily, coating integrity can be sacrificed for higher biocompatibility. However, for more permanent long-term purposes where highly crystalline HA phase is necessary, the use of post-treatment techniques is required to restore biocompatibility that is otherwise absent in a mechanical adherent and amorphous as-sprayed coating.
REFERENCES 1 2
3 4
5
6
Kay JF. Calcium phosphate coatings for dental implants. Dental Clin North Am 1992; 36(l): l-18. Cahn RW, Haasen P, Kramer EJ. Material Science and Technology, Medical and Dental Materials, Vol 14. New York: VCH, 1993: 278-279. Jacho M. Calcium phosphate ceramics as hard tissue prosthetics. Clin Orthop 1981; 157: 259-278. De Groot K, Klein CPAT, Wolke JGC, De BlieckHogervorst JMA. Handbook of Bioactive Ceramics, Vol. II. Boca Raton, FL: CRC Press, 1990: 3-17. River0 DP, Fox J, Skipor AR, Urban RM, Galante JO. Calcium phosphate coated porous titanium implants for enhanced skeletal fixation. J Biomed Mater Res 1988; 22: 191. Ducheyne P, Van Raemdonck W, Heughebaert M.
and K.A.
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Structural analysis of hydroxyapatite coatings on titanium. Biomaterials 1986; 7: 97. 7
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Lacefield WR. Bioceramics. In: Ducheyne P, Lemons JE eds. Materials Characterization versus In-Vivo Behaviour, New York: New York Academy of Sciences, 1988; 72-80. Asahi Optical Co Ltd. Coating of prosthetic materials by hydroxyapatite sputtering. Japanese Patent 58, 109, 049, 29 June 1983. Heide H, Roth U. Pressure bonding of hydroxyapatite coatings onto titanium implants. Proceedings of the International Conference on Hot Zsostatic Pressing, Centek Publishers, Lulea, Sweden. 1987: 423-427. Gerdeman DA, Hecht NL. Arc Plasma Technology in Material Science. NY, USA: Springer-Verlag, 1972. Malmberg S, Heberlein J. Effect of plasma spray operating conditions on plasma jet characteristics and coating properties. J Thermal Spray Technol 1993; Z(4): 339344. Manley MT, Kay JF, Yoshiya S, Stern CS, Shilberg BN. Accelerated fixation of weight bearing implants by hydroxyapatite coatings. Trans 33rd Meeting Orthopedic Research Society 12, 1987: 214. Khor KA, Cheang P. Effect of powder feedstock on thermal sprayed hydroxyapatite coatings. In: Berndt CC, Sampath S, eds. Thermal Spray Industrial Applications Thermal Spraying Conference NTSC ‘94. OH: ASM International, 1994: 147-152. Zyman Z, Weng J, Liu X, Zhang X, Ma Z. Amorphous phase and morphological structure of hydroxyapatite plasma coatings. Biomaterials, 1993; 14(3): 225-227. Radin S, Ducheyne P. The effect of plasma sprayed induced changes in the characteristics on the in vitro stability of calcium phosphate ceramics. J Mater Sci 1992; 3: 33-42. de Groot K. Ceramics of calcium phosphates: preparation and properties. In: de Groot K, ed. Rioceramics of Calcium Phosphate. Boca Raton, FL: CRC Press, 1982. Gross KA, Berndt CC. Structural changes of plasma sprayed hydroxyapatite coatings during m-vitro testing. In: Horowitz E, Parr JE, eds. Characterisation and Performance of Calcium Phosphate Coatings Implants, Philadelphia, PA: ASTM, 1993: 256-268.
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Maxian SH, Zawadsky JP, Dunn MG. Mechanical and histological evaluation of amorphous calcium phosphate and poorly crystallised hydroxyapatite coatings on titanium implants. J Biomed Mater Res 1993; 27: 717-728. Klein CPAT, Wolke JGC, de Blieck-Hogervorst JMA, de Groot K. Calcium phosphate plasma-sprayed coatings and their stability. J Biomed Mater Res 1994; 26: 909-917. Chen J, Wolke JGC, de Groot K. Microstructure and crystallinity in hydroxyapatite coatings. Biomaterials 1994; 15(5): 396-399. Tagai H, Aoki H. In: eds. GW Hastings and DF Williams, Mechanical
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Khor KA, Cheang, P. Hydroxyapatite powders and coatings produced by thermal spray techniques. Proceedings of the 2nd International Symposium on Apatite, 3-6 July 1995, Tokyo, Japan, ed H. Aoki (in press). Vardelle M, Vardelle A, Fauchais P. Study of the trajectories and temperatures of powders in a DC. plasma jet - Correlation with alumina sprayed coatings. 30th International Thermal Spraying Conference, Deutscher Verlag fur Schweisstechnik (DVS), Dusseldorf, Germany, 80,1983: 88-91. Gross KA. Surface modification of prosthesis. Thesis. Australia: Monash University, 1990. Biomaterials 1996, Vol. 17 No. 5
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Khor KA, Cheang P. Characterization of thermal sprayed hydroxyapatite powders and coatings. I Thermal Spray Technoll994; 3(l): 45-50. Cheang P. The microstructural and mechanical properties of plasma sprayed coatings. Thesis. Australia: Monash University, 1989. Ravaglioli A, Krajewski A. Bioceramics: Materials, Properties and Applications. NY, USA: Chapman & Hall, 1992: 264. Khor KA, Cheang P. J. Mater Proc Technol 19% (submited). Cheang P, Khor KA. Surface modification of plasma sprayed bioceramic coating by pulsed laser. In: Goh
Biomaterials 1996,Vol. 17 No. 5
with
plasma
30
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spraying
of HA coatings:
P. Cheang and K.A. Khor
JCH, Nather A, eds. Proc. 8th International Conf. an Biomedical Engineering, Singapore, 7-11 December, NLJS: Singapore, 1994: 531-533. Khor KA, Cheang P. Plasma spraying of zirconia reinforced hydroxyapatite coatings. In: Goh JCH, Nather A, eds. Proc. 8th International Conf. on Biomedical Engineering, Singapore, 7-11 December, NUS: Singapore, 1994: 536-538. Yip CS, Khor KA, Loh NL, Cheang P. Microstructure and properties of hot isostatically pressed bioceramic composites. Proceedings of an Int. Conference on Mechanics of Solids and Materials Engineering. MSME-95, Vol A, NTU, Singapore, pp.191-195, 1995.
17 (1996)
537-544
published by Else&x Science Limited Printed in Great Britain. All rights reserved
0
1996
ELSEVIER
0142-9612/96/$15.00
Addressing processing problems associated with plasma spraying of hydroxyapatite coatings * P. Chean& and K.A. KhorS t School of Applied Science and t School of Mechanical and Production University, Nanyang Avenue, Singapore 2263, Singapore Biomedical
coatings
linity (for positive necessary Thermal
generally
biological
to enhance
have to satisfy specific
responses),
spray processes
such as hydroxyapatite
implants
have been widely
generated
concerns
investigations on coating performance
acknowledged,
using HA coatings
such as bioresorption, such as powder
degradation
morphology
In addition,
conditions
tive or secondary
treatment
problems
associated
feedstock
morphology
efficiency
and produce
Keywords:
certain
opposing
via thermal
stages
with plasma
to attain the desired spray coating
suitable
an acceptable
coating
structure.
plasma
sprayed
spraying,
composition
Variation
has Recent
has significant
influence and
in process
and mechanical
inconsistencies
In order to reach some acceptable in commercially
results.
available
the means to achieve
creating
This paper
powder
highlights
that tailoring processes
properties,
level
HA the necessary
the need to introduce
conditioning
microstructure,
HA coatings.
variability
of HA and suggests
through
in coated
poor performances
and chemical
constrain
alone; therefore,
and properties
Hydroxyapatite,
related
variability
factors severely
spraying
are
fixation.
of HA materials
of several
and bone apposition.
to control existing
it may be necessary
and improve
active biomedical
can induce microstructural
of reliability,
of crystal-
These
functionally
structure
of the coating.
porosity.
The benefits
of thermally
have shown that process
such as phase composition,
performance
coating
implants.
and reliability
such as a high degree healing
used to deposit
but the occurrence
Nanyang Technological
and optimal
post-operative
that have an effect on the service feedstock.
adhesion
(HA), onto prosthetic
over the consistency
characteristics
parameters
accelerate
have been frequently
coatings,
requirements
good coating
biocompatibility,
Engineering,
other innova-
some of the
the powder
can aid the deposition
bioactive
phases
Received 9 March 1995; accepted 15 May 1995
adverse reaction by the provision of a biocompatible phasel. Some important coating characteristics that are generally expected of biomedical coatings are good biocompatibility of the desired phase and crystallinity, good coating integrity and adequate porosity. Phase and crystallinity are important to ensure that the appropriate biological responses occur. The formation of transitional phases can elicit unfavourable resorption and toxicity. Coating integrity is needed to ensure that necessary mechanical characteristics are present to keep the coating intact. Porosity of the optimum size can improve fixation by promoting bone ingrowth to create mechanical interlocking’. A bioceramic commonly used as coating material for prosthetic implants is HA. It is a bioactive material with chemical composition similar to natural bone. It has been known to form strong biological bonds with bony tissue without the presence of soft fibrous tissues3’4. The provision of a high calcium and phosphorus-rich environment promotes rapid bone formation within the vicinity of the implant. HA also
Coatings used for biomedical applications are often subjected to many stringent constraints. In general, materials that are applied onto metallic prostheses often adopt a secondary role in enhancing the characteristics of the implants. As coatings, they are not intended to substitute existing materials but more correctly to improve the characteristics of a fully functional implant’. Good prosthetic designs normally have provisions to accommodate eventualities such as coating failure without jeopardizing the function of the implant. This perception is important to ensure that the role and purpose of coatings are not misunderstood. Coatings have specific functions ranging from improving fixation by establishing strong interfacial bonds, shielding the metallic implant from environmental attack or leaching effects, promoting fast tissue growth and interaction by the presence of a catalyst material, hydroxyapatite (HA), and minimizing *Presented in part at the 8th International Conference on Biomedical Engineering, 7-10 December 1994, Singapore. Correspondence to Dr K.A. Khor. 537
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establishes strong interfacial bonds with titanium implants. Its excellent biointegration makes HA an ideal choice for use in orthopaedic and dental applications. HA coatings can be applied in several ways4-g. The use of thermal spraying techniques to coat HA onto implants is popular due to certain advantages such as simplicity, high deposition rates, low substrate temperature, variable coating porosity, variable phase and structure. With this process, any material that does not decompose or vaporize during melting is suitable for thermal spraying’“. Particulate materials are injected into the high temperature heat sources where they are melted and propelled at high velocity towards a substrate. The molten droplets flatten on impact as splats to form a lamellar coating structure. Although thermal spraying is conceptually a simple process, it is, however, complex in operation. Some 100 variables are associated with the spraying process which have interrelated influence on the microstructure and properties of the final coatinglo. Among these are the operating conditions of the plasma spray system (power setting, choice of auxiliary gas, feed rate, etc.), the size of the plasma jet hot zone and the heat and momentum transfer from the plasma jet to the particlesl’. The turbulent profile of the plasma jet affects the quantity of air entrainment and, hence, the heat transfer rates to particles travelling through the plasma process zone. While it is easy to form any coating, it is difficult to produce a coating with the desired coating characteristic. Furthermore, there is generally a concern over the consistency and reliability of the coating quality. This is particularly true for HA coatingsl’. Recent studies have concentrated on the selection of spraying parameters that affect the coating microstructure and properties, hence coating performance. The influence of the morphology and denseness of the feedstock was also investigated13. A problem pertinent to the plasma-sprayed HA coating is the generation of an amorphous phase, along with other non-bioactive calcium phosphate phases following plasma spraying14. The formation of the amorphous phase is apparently associated with partial dehydroxylation of the HA during the plasma spray process15. The presence of an amorphous calcium phosphate phase is undesirable because natural bone HA is crystalline, thus the integrity of the bone-implant interface is compromised16. In addition, the strong resorption of the amorphous phase may cause mechanical and adhesive instability in the coating. Degradation of HA coatings with high amorphous phase content occurs by d&-adhesion of cracked lamellae and dissolution of the remaining lamellae during in vitro tests17. Mechanical tests show that failure of the bone-coating-implant interface consistently occurred within the bone for amorphous and poorly crystallized HA coatings as a result of bioresorptionl’. Some factors identified as influencing the coating stability in plasma-sprayed HA coatings during in vivo experiments include the particle size distribution, degree of melting (determined by the feed position of the powders into the plasma flame) and post-spray treatmentlg* “. As results from recent studies have indicated, thermal spraying of HA coating is indeed a formidable challenge to a materials Biomaterials 1996, Vol. 17 No. 5
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plasma
spraying
of HA coatings:
P. Cheang
and K.A.
Khor
processor due to the low degree of ‘forgiveness’ of both the process and the material concerned. The plasma spray process demands a series of conditions such as appropriate particle size range, power setting, particle morphology, etc., for optimal coatings, while the material concerned, HA, reacts strongly to rapid solidification following the plasma spray and yields a series of amorphous and metastable phases. In this study, the problems encountered during the thermal spraying of HA-based materials are examined. Particular emphasis is placed on the characteristics of the powder feedstock and their influence on the microstructural and mechanical properties of plasmasprayed HA-based coatings.
EXPERIMENTAL TECHNIQUE The HA feedstock was prepared by wet reacting orthophosphoric acid with calcium hydroxide in specific mole concentratior?. The fine precipitates were later used to produce three other powders: calcined HA (CHA), spray-dried HA (SDHA) and spheroidized HA (SHA). CHA was produced by oven drying at -180°C and calcining the feedstock at 800°C for 4 h. SDHA was prepared by blending HA with 2% organic binder and spray dried using a two-fluid nozzle (Lab-Plant SD-04 system, UK). The size range of the SDHA is approximately 2-20 pm. SHA was produced in-house by flame spraying the 53-75 pm CHA faction using a combustion flame gun PF-73 (Miller Thermal Inc., USA) into distilled water. The HA coatings were produced via plasma spraying using a 40 kW DC plasma torch (SG-100 Miller Thermal Inc., USA) and assisted by a computerized closed loop rotor-feed hopper. Typical spraying conditions are tabulated in Table 1. Optical light (Nikon Metallurgical System) and scanning electron microscopy (Cambridge S360 SEM) were used to evaluate the surface morphology and sectioned microstructure of coatings and powders. A powder X-ray diffractometer system (Philips MPD 1880) was used to determine the phase composition of the coatings.
Table
1
Plasma spraying
Main arc gas (Ar) Auxiliary gas (He) Arc current Arc voltage Spraying distance
’ 1 psi
parameters 50 psi” 15-50 psi 800 A 32-35 V 120 mm
= 6.9 kPa.
RESULTS Particle morphology Calcined HA (CHA) The CHA powders are essentially angular in shape with a porous structure comprising fine agglomerated particles with size 2-5 pm. Particle size between 53 and 75 pm were used for plasma spraying (Figure 2).
Problems
associated
with plasma spraying
of HA coatings: P. Cheang and K.A. Khor
Percent~gc I%
539
Plasma Vs Flame Spheroidization of HA
__
30
20
10
0 10
20
32
-
Flame
m
Figure 4 Particle size distribution spheroidized hydroxyapatite. Figure 1
Calcined hydroxyapatite
53 Particle
45
siu
76 /microns
Plasma
of plasma
and flame
powders.
Spray dried HA (SDHA) The spray-dried powder sizes range between 2 and 20 pm and is similar in structure to CHA except for its spherical geometry (Figure 2).
Spheroidized HA (SHA) These are highly dense spherical particles with a glassy smooth surface texture. Particle sizes range between 20 and 53 pm (Figure 3). Comparison
a Figure 2
Spray-dried
hydroxyapatite
powders.
b Flgure 3
Flame spheroidized
hydroxyapatite
powders.
Figure 5 a, As-sprayed surface of calcined hydroxyapatite coating; b, cross-section of calcined hydroxyapatite coating. Biomaterials 1996, Vol. 17 No. 5
Problems
540
associated
with
plasma
spraying
of HA coatings:
P. Cheang
and K.A.
Khor
led to deposition and phase inconsistency caused by the impact of irregularly sized particles. Completely different surface and cross-sectional microstructures were observed using SHA powders (Figure 7). The regular formation of neatly stacked disc-like splats produced a flat and smooth surface profile. The cross-sectional view of the coating microstructure reveals a lamellar structure that is free of large macrovoids. The presence of good interlamellar contact and the absence of macropores and unmelted particles greatly improves coating integrity.
X-ray diffkaction (XRD) analysis XRD analysis of the plasma-sprayed CHA coatings revealed the presence of amorphous calcium phosphate, tetracalcium phosphate (TET), tricalcium phosphate (TCP) and calcium oxide (CaO) in addition to crystalline HA. They differed considerably from the starting material that was essentially pure and crystalline HA. Comparing the phase content of the SDHA coatings, it was observed that both the crystallinity of
b Figure 6 a, As-sprayed apatite; b, cross-section coating.
surface of spray-dried hydroxyof spray-dried hydroxyapatite
between plasma spheroidized and combustion flame SHA powders have shown that the latter batch possessed a more uniform distribution (Figure 4) and consistent chemical composition among the particles (determined using energy dispersive X-ray analysis)“.
a
Coating microstructure Using angular CHA powders produced relatively poor coating microstructures. They consisted of micro- and macro-pores, irregular splats and unmelted particles with limited interlayer adhesion (Figure 5). Considerable amounts of unmelted particles were present which contributed to the formation of large pores and cavities. These coatings have poor structural integrity because of limited interparticle cohesion. Some microstructural improvements were observed using SDHA powders. The reduction in the overall porosity was attributed to better flow behaviour of the spherical powder (Figure 6). However, the weakly agglomerated structure of both powders did not eliminate the problem of particle break down which Biomaterials
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b Figure 7 a, As-sprayed surface of spheroidized hydroxyapatite coating; b, cross-section of spheroidized hydroxyapatite coating.
Problems
associated
with
plasma
spraying
of HA coatings:
Calcined
HA
(53-75
JU,I)
(45-53
Spray
Z-Theta
Figure 8 X-ray diffraction spray-dried hydroxyapatite tite coatings.
Dried
HA
(5-20
P. Cheang
pm)
pm)
Angle
of calcined hydroxyapatite, and spheroidized hydroxyapa-
HA and peak intensity of CaO were lower (Figure 8). For SHA, the peak intensity of HA was similar to CHA, although some degree of line broadening was observed. The level of CaO was found to be the lowest among the three powders.
DISCUSSION Thermal spraying processes involve complex interactions between the heat source, particulates and substrate. Particles injected into the plasma zone experience extreme heating rates and temperatures exceeding 10 000”Cz3. As a result, the majority of the powder, within a critical size range, melts to yield an aerosol of molten droplets. Large particles, above the critical size range, may experience only partial melting while the small particles may have a layer of vaporized material surrounding the liquid droplets. Hence, conditions arising from plasma-particle interactions such as short residence time (milliseconds), high gas velocities and inconsistent trajectory path of different particle sizes generally cause incomplete and nonThese heated uniform treatment of particulates. particles further interact with the surrounding atmosphere and subsequently impact at high velocity and cooling rate (about 10” KS-‘) to form a coating. The solidification process of the molten droplet is obviously non-equilibrium. These conditions promote the formation of metastable phases and a complex microstructure comprising macro- and micro-porosity, fused and partially molten particles that have limited
and K.A. Khor
541
interparticle cohesion. In general, the deposition profile of the as-sprayed coating depends largely on the operating state of the plasma and the physical characteristics of the powder. These ultimately introduce large variability that may or may not be desirable, depending on the application concerned. However, proper understanding and selection of essential parameters could minimize unfavourable properties. Investigations into the thermal spraying of HA coatings have shown that variation in operating parameters can cause significant changes to the microstructural and mechanical properties of the coating. Even though similar starting materials were used, process-related factors have significant effects on the final coating characteristic. XRD analysis of both powder and coating reveal distinctly different peak profiles. Variation in the powder characteristic in terms of morphology, structure and size alters the coating characteristic in terms of integrity, microstructure, surface profile and degree of crystallinity. This therefore suggests that in order to achieve coating consistency, hence similar mechanical and biological behaviour, similar powder type and configuration must be used, provided all other parameters remain constant. Achieving this in practice is difficult because of the wide variability of commercially available powdersZ4. Coating generated from agglomerated spray-dried powder has differing characteristics from that of densely fused powders. The variation in the coating microstructure is caused by differing stability of the powder, flow characteristic, melting behaviour, deposition rate and droplet size. The stability of the feedstock affects the flow behaviour, deposition consistency, phase content and degree of crystallinity in the as-sprayed coating. The flow characteristic of the powder is dependent on factors such as particle size and size distribution. Finer particles have greater difficulty to flow consistently due to closer particle spacing and static effects. This would lead to intermittent feeding, inconsistent deposition and coating unevenness. Powders with a narrow particle size range have better flow behaviour compared to mixed sizes comprising large and fine particles. Fines generated from a weakly bonded particle also disrupt flow. Under the turbulent environment of the plasma jet, these weakly agglomerated particles are likely to break down to create further complications during deposition. The physical nature of the particle has an effect on the melting characteristics. Therefore, differences in melting behaviour will occur between dense and porous particles. Porous particles have the tendency to form hollow spheres that further increase coating porosity. With dense particles, the formation of a filled spherical droplet would lead to a more uniform splat pattern. Quality coatings that are both smooth and dense are normally formed by the deposition of monosized particles that are sufficiently molten. The sizes of the impacting particles have an effect on the phase composition. The thermal state of the particle is governed by the heat absorbed during plasmaparticle interaction. Some factors influencing the Biomaterials
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degree of heat absorbed are temperature, heating rate, residence time, particle conductivity, thermal geometry, melting point, heat of fusion and density. For a constant plasma environment, a smaller particle size is expected to reach higher temperature. The ease with which materials melt during thermal spraying is defined by the difficulty of melting factor (DMF). This is also related to the maximum fusible particle size (SC). This suggests that for any plasma condition and material system, there is a critical particle size that generates optimum coating quality. Deviation from this optimum value would degrade the quality of the coating by introducing excess porosity, transitional phases and amorphous content. However, the critical value for optimum biocompatibility and mechanical integrity may not necessarily be the same. Previous investigation with agglomerated HA powders has shown that the percentage of unmelted crystalline phases increases with increasing particle sizes above Scz5. Below this value the degree of amorphous nature and transitional phases increased with decreasing particle size. But a more coherent coating microstructure is formed. Thus, in order to achieve a high degree of crystallinity in the as-sprayed coating, a size slightly larger than the largest fusible particle size (SC) must be used. However, too large a size would introduce excessive porosity that could weaken the coating mechanical properties. Since the size of impacting particles severely affects both phase and crystallinity, the stability of the particle during plasma-particle interaction becomes important. To ensure that the optimal thermal states are achieved during heating, the geometry of the particles must be maintained. Investigation with weakly agglomerated HA powders has shown that particle breakdown occurs to an extent that a complex multiphase coating emerges that has varying degree of crystallinity, This inadvertently introduces undesirable properties such as poor biocompatibility and mechanical integrity. Thermal sprayed ceramic coatings are generally porous. This characteristic was initially thought to benefit biomedical application because of the possibility of mechanical fixation through bony ingrowth. Porosity inherent of thermally deposited coatings are essentially bimodal in nature. Pores exist as large cavities above 10 pm to fine pores of less than 1 pm. Fine pores are generally considered as interlamellar pores that occupy 70% of the overall porosity’“. As thermally sprayed coatings are built up of successive layers of lamellar splats, the degree of good contact between lamellae greatly affects the quality of the coating bond strength. This unique arrangement of lamellae renders the coating stiffer in the planar direction as opposed to the perpendicular direction. This anisotropic behaviour is advantageous to applications subjected to shear forces as is normally experienced in the stem section of hip implants. However, pores that normally promote bone ingrowth are generally in the ZOO-GOO pm rangeZ7. The formation of such pores is not encouraged because of the severely weakening effects on mechanical properties. Considering the requirements of a biomedical
with plasma
spraying
of HA coatings:
P. Cheang
and K.A. Khor
coating, it appears that some factors are difficult to achieve while others are conflicting in nature. Thermal spraying invariably causes physico-chemical changes to the starting material, Good biocompatibility requires that the coating material contain a highly crystalline form of HA that is bioactive. The other forms of transitional phases such as TET, TCP and amorphous calcium phosphate have differing biologiAs previously mentioned, highly cal responses. crystalline HA is achieved by using a particle size that is slightly larger than the critical melting size. With larger size, incomplete melting occurs which retains the original crystalline form of HA. On impact, semi-molten particles interlock mechanically to form a complex array of pores and irregularly shaped compacts. Because of the solid nature of the particles, and the high degree of porosity, the mechanical properties are generally poor. Having good structural integrity, on the other hand, requires a regular stream of adequately molten droplets that spread in an orderly fashion on impact, without fragmenting, to form a uniform lamellar structure with minimal porosity. Good adhesive and cohesive properties can be expected from this deposition. In fact, recent tensile adhesion test results support this sensitive dependence of mechanical properties on microstructure. The bond strengths of porous CHA coatings are lower than densely packed SHA coating?. However, well-melted coatings are generally amorphous and contain transitional phases that are more bioresorbable. Here, the mutually opposing effects of biocompatibility and mechanical integrity are clearly demonstrated. Although the long-term benefits of a permanent biocompatible coating are not known, short-term advocators of the HA coating to induce rapid bone growth and fixation emphasize the greater importance of biocompatibility over mechanical properties. Should a more permanent coating be required over extended periods then the engineering of a more coherent coating becomes crucial and obviously more difficult. The constraints introduced by opposing effects of both coating integrity and biocompatibility limits the possibility of achieving the desired coating characteristics in a single-step spraying process. Certainly the use of post-treatment processes involving thermal and laser techniques provides greater opportunity to attain the necessary requirements. Preliminary investigation of laser-treated coatings favours the heat treatment processing to induce phase transformation rather than melting for densification because of apparent crackingzg. A recent investigation using HA-based composite powders suggest a composite coating containing a bioinert ceramic and a bioactive or bioresorbable HA”‘. Such a technique would exploit the strength of the spraying process for melting and deposition and at the same time achieve its dual characteristic based on powder design. Spheroidized composite powders containing two different materials have already been produced and these coatings are currently under investigation. Using hot isostatic pressing to further enhance the coating property is certainly an avenue worth considering31.
Problems
associated
with
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spraying
of HA coatings:
P. Cheang
CONCLUSIONS The problems encountered with the use of thermally sprayed HA coatings are mainly improper melting of the feedstock, reproducibility and satisfying biomedical requirements. Mixed successes with the use of HA coated implants generates a level of uncertainty that the material is unpredictable. This investigation from the purely processing perspective suggests that suitable powder conditioning can help alleviate some of the existing variability originated from a lack of standardization of operating parameters. HA feedstocks that are used for thermal spraying vary considerably from composition, phase, crystallinity, particle size, shape and structure. Most HA powders are of the agglomerated, spray-dried genre. These powders, unfortunately, have a tendency to break down in the turbulent plasma and result in a coating that is built up of droplets whose size range is a mixture of the original size range and broken down fragments. In addition, previous work on plasma spraying of HA coatings utilized powder sizes that ranged between 1 and 100 pm. This will give rise to a wide array of plasma-particle reactions. The resultant coating from this wide particle size range will not provide the level of consistency that is desired. Considering that the coating microstructure is highly dependent on powder characteristics, this alone is sufficient to cause variation. The selection of particle size without considering the critical particle size is certain to lead to the formation of unfavourable weak amorphicity and mechanically phases, microstructures. Satisfying the requirement for pore sizes in the range 200-400 pm is difficult with thermal spraying. Additional post-treatment processes Both biocompatibility and coating are necessary. integrity are competing factors that are mutually exclusive. For short term application of HA coating where the coating exists temporarily, coating integrity can be sacrificed for higher biocompatibility. However, for more permanent long-term purposes where highly crystalline HA phase is necessary, the use of post-treatment techniques is required to restore biocompatibility that is otherwise absent in a mechanical adherent and amorphous as-sprayed coating.
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3 4
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6
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JCH, Nather A, eds. Proc. 8th International Conf. an Biomedical Engineering, Singapore, 7-11 December, NLJS: Singapore, 1994: 531-533. Khor KA, Cheang P. Plasma spraying of zirconia reinforced hydroxyapatite coatings. In: Goh JCH, Nather A, eds. Proc. 8th International Conf. on Biomedical Engineering, Singapore, 7-11 December, NUS: Singapore, 1994: 536-538. Yip CS, Khor KA, Loh NL, Cheang P. Microstructure and properties of hot isostatically pressed bioceramic composites. Proceedings of an Int. Conference on Mechanics of Solids and Materials Engineering. MSME-95, Vol A, NTU, Singapore, pp.191-195, 1995.