Effects of gravity on combustion synthesis of functionally graded biomaterials

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doi: lO.l016/SO273-1177(03)00392-2

EFFECTS OF GRAVITY FUNCTIONALLY

ON COMBUSTION SYNTHESIS OF GRADED BIOMATERIALS

M. Castillo’, J. J. Moore’, F.D. Schowengerdt’, R.A. Ayers’, X. Zhang’, M. Umakoshi’, H.C. Yi* and J.Y. Guigne2 ‘Center for Commercial Applications of Combustion in Space (CCACS), Colorado School of Mines, 1500 Illinois St., Golden, Colorado 80401, U. S. A. ‘Guigne’lnternationai Ltd, 685 St. Thomas Line, Paradise, NewfoundlandAlL ICI, Canada ABSTRACT Combustion synthesis, or self-propagating, high temperature synthesis is currently being used at the Colorado School of Mines to produce advanced materials for biomedical applications. These biomaterials include ceramic, intermetallic, and metal-matrix composites for applications ranging from structnral to oxidation- and wear-resistant materials, e.g., Tic-Ti, TiCCrrC2, MoSir-Sic, NiAl-TiBr, to engineered porous composites, e.g., B&-A&OS, TiTiB,, Ni-Ti, Car(PO& and glass-ceramic composites, e.g., CaO-Si02-BaO-A&Or-T&. The goal of the functionally graded biomaterials project is to develop new materials, graded in porosity and composition, which will combine the desirable mechanical properties of implant, e.g., NiTi, with the bone-growth enhancement properties of porous biodegradable ceramics, e.g., Ca@O& . Recent experiments on the NASA parabolic flight (KC-135) aircraft have shown that gravity plays an important role in controlling the structure and properties of materials produced by combustion synthesis. The results of these studies, which will be presented at the conference, will provide valuable input to the design of experiments to be done in Space-DRUMSTM, a containerless materials processing facility scheduled to be placed on the International Space Station in 2003.0 2003 COSPAR.Published by Elsevier Ltd. All rights reserved. INTRODUCTION Functionally graded materials (FGM) offer the advantage to engineer materials with specific structural, compositional, morphological, and mechanical properties. This concept is being increasingly applied to biomaterials as the demand for improved advanced porous implant material increased. A .diverse amount of materials have been considered for such bone replacement/scaffold materials. The idea of combining these materials to create a functionally graded material is relatively new and discussedby Peters et al. (2000). The method of self-propagating high temperature (SHS) is viewed as beneficial in creating such advanced materials. This process exploits the exothermicity of the reactants and the ability to create a highly porous structure using able biocompatible/bioactive materials. SHS has the advantage to manipulate the pore structure, morphology, and combustion temperatures in order to achieve a functionally graded advanced material. MATERIALS

& METHODS

SHS reaction systems can be determined using a thermodynamic balance of heat capacities that correspond to specific phasesand associatedphase transition enthalpies as discussedby Moore (1994). This is performed using a temperature-enthalpy diagram as shown below in Figure 1 to determine the adiabatic temperature. It has been shown that SHS reaction systemswith adiabatic temperatures greater than -2000 K will react through a propagating mode provided that there is not a significant amount of heat loss. Systemsthat have lower reaction temperatures and systemsthat dissipate heat too rapidly will not react in propagating mode but may react using other methods such as the simultaneous combustion mode. Adv. Space Res. Vol. 32, No. 2, pp. 265-270.2003 8 2003 COSPAR. Published by Elsevier Ltd. All rights Printed in Great Britain 0273-l 177/$30.00 + 0.00

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A H ( T i gI= -EH(PbH(RII

<.....

To

TI

Tig

‘I’, (To)

Tad tTojTad

CT,) T,d

tTig)

Temperature (K) Fig. 1. Theoretical temperature

enthalpy diagram used to determine

successful SHS reaction systems.

The SHS reaction is initiated using a localized heat source (e.g. tungsten coil) once the reactants have reached the ignition temperature (Tig). The reaction will propagate through the unreacted material delivering reaction enthalpy to the layers ahead of the propagating reaction. This will also achieve the combustion temperatnre (T,), thus self-sustaining the exothermic reaction throughout the reaction mixture. The heat loss (AQ) from the reaction front corresponding to the difference between the measured T, and the calculated T,,dis indicated in Figure 1. This figure indicates that the total enthalpy that contributes to the SHS reaction corresponds to H(P). A typical SHS process includes 1) mixing of reactant powders, 2) forming of pellet by uniaxial or preferably isostatic pressing, 3) loading into the combustion chamber, and 4) ignition of the combustion reaction. SHS reactions take place on the order of seconds and are associatedwith heating rates up to 2000%. All samples were pressed into cylinders (dia =1.27cm., h=1.27-2.lcm) and ignited via a tungsten coil in an argon atmosphere. Reaction systems comprising combinations of CaO and P205 require that all mixing, pressing, and test reactions occur in a high purity inert atmosphere (i.e. glovebox) on account of the hygroscopic nature of PzOs. The SHS reaction systemsto be reported in the paper include: 3CaO + PzOs= Ca,(PO& (x+y)Ti + xB = yTi + xTiB Ni + Ti = NiTi (l+x)Ti + C = TiC + xTi

(1) (2) (3) (4)

SHS experiments were conducted in low gravity through the NASA KC-135A Reduced Gravity Research Program. Parabolic flight patterns are used to obtain -20 seconds of low gravity and 40 parabolas,perday are generally flown. A special rack (COSYM) is used to perform the SHS experiments aboard the plane. Temperature, video, and pressuredata are obtained together with the production of samples. Processparameters include green density, particle size, gasifying agents, composition, and gravity. Each of these processing parameters affect the porosity, amount of interconnected pores, and pore shape to facilitate the engineering of SHS-produced materials with specific porosities as well as the construction of functionally graded porosities. Scanning electron microscope (SEM) images were produced using a JEOL JXA-&IO SEM. The SEM analysis was coupled with a Therm0 NORAN Lithium drifted 10mmz Electron Dispersive X-ray (EDX). The samples were

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coated with gold for SEM analysis. X-ray Diffraction (XRD) analysis was performed with a Philips X’Pert MPD Pro Theta/Theta X-ray diffraction system. The microstructure of the TiB-Ti samples was studied with an Olympus SZX 12 stereoscope. RESULTS SEM photomicrographs of the Car(PO& based materials are shown below in Figure 2 and indicate the difference in microstructure. Figure 2(A) is a photomicrograph of a sample reacted in.low gravity and partially cooled in a low gravity and -2 g environment (due to the length of the parabola in the flight pattern of the KC-135). The grains exhibit the typical grain growth phenomenon of six sided grains growing at the expense of those growing with fewer facets. Figure 2 (B), shows grains that have cooled in a Ig environment in which the microstructure exhibits lamellar grains with characteristic spots.

Fig. 2. SEM images bf Ca3(PO&

produced in low gravity (A) and at 1 g (B).

EDX analysis was taken from the center of both grains shown in Figure 2. The low gravity sample produced a calcium to phosphorus ratio of 1.07 while that produced in Ig exhibited a 0.47 calcium to phosphorus ratio. The EDX/SEM samples were coated in gold, therefore deconvolution of the phosphorus and gold peaks was performed. XKD analyses of samples produced under low-gravity and 1 g are shown in Figure 3. Both spectra match the alpha phase for tricalcium phosphate (PDF 70-0364). Valdes et al. has determined that the characteristic spots similar to that shown in Figure 2 (B) are calcium pyrophosphate (CarPrOr). It should be noted here that the latter is a bulk analysis and the above EDX analysis is a microanalysis.

lo

20

Fig. 3. XRD spectra for Cas(PO&

30 40 2-Theta (*)

50

60

produced in low gravity and 1 g conditions.

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The effect of gravity on the (x+y)Ti + xE%= yTi + xTiB is shown in Figure 4. The longitudinal or propagating direction is shown below with ignition from the lower side (bottom of sample). Sphere-like pores were produced in low gravity environments and radial pores were produced under terrestrial conditions.

1, Propagating Direction

Fig. 4. Effect of gravity on reaction system’(x;y)Ti, + XB = yTi + xTiB. Tie k-produced materials are 92% TiB and 8%Ti. (A) was produced in low gravity aboard the KC-135 and (8) was produced in terrastrial(1 g) conditions.

XRD analysis of NiTi produced via SHS is given in Figure 5. NirTi and NiTir were also formed with the equiatomic NiTi in the combustion process. This is ofien the case for commercially produced NiTi.

1600 1400 1200

p 1000 g 800 E 600

,I= NbTi

20

40

-. - ._-- --..Fig. 5. XRU of SHS

60

80

28 . . ..-. . produced NI I I under

100

120 .. .

140

....

terrestrtai conaeons.

The superelastic nature of the as synthesized NiTi system is demonstrated below in Figure 6. The hysterisis curves indicate that as the compressive stressincreasesthe heat treatment will allow more recovery of compressive strain. Although the result is not shown, the martensitic transformation for this material was seen to be 47°C. The shape memory effect is governed by martensitic transformation and amount of material that transforms.

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6ook

Q500 $I00 I %oo g ‘Gi m &200 g UlOO 0 Fig. 6. Stress-Strain

2 Comprcssiv4cStrain @-&I6 hysterisis curves for the NiTi sytem demonstrating

the shape memory effect.

The (l+x)Ti + C = TiC + xTi reaction system was investigated under terristrial to produce a “self-assembled” functionally graded material due to the effect of gravity-driven fluid flow. Figure 7 below shows a SEM photomicrograph of the graded material. The top region has an apparent porosity of -70% and contains -100% of the TiC phase. The bottom region has an apparent porosity of -30% and is composed of -50 atomic% TiC and -50 atomic% Ti. 70% Porosity 100% TiC

T Increasing TIC

30% Porosity 50% TIC - 50% Ti system 4 (TiC-Ti). 1 ‘he top layer is composed of Fig. 7. SEM photomici rogr aph 01f FGM SHS produced= -100% TIC Fchase and the low er section is composed of -50 atomic% of TIC phase and -50 atomic% of the Ti phase.

DISCUSSION & CONCLUSTIONS Gravity plays a significant role in the structure and properties of materials produced by SHS. In particular, different gravity environments have a great effect on the Car(PO& microstructure produced by SHS as shown in the SEM photomicrographs in Figure 2. Low gravity produces classic grain growth phenomenon while the terrestrial environment yields long lamellar grains with characteristic spots. EDX analysis shows that the calcium

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to phosphorus ratio is 1.07 for grams produced in low g while the calcium to phosphorus ratio is 0.47 for the sample produced under terrestrial (1 g) conditions. The samples produced on the KC-135 were partially cooled in low gravity (-0 g) and high gravity (-2 g) conditions due to the parabolic flight path timing of the KC-135. Bulk analysis performed with XRD (Figure 3), showed that both Ca@O& samples produced in low gravity and 1 g formed the alpha phase of tricalcium phosphate. It has been shown by Gortcheva et al. (1997) that the characteristic spots seen in Figure 2 (B) are calcium pyrophosphate (Ca2Pz0,). The microstructure studied at the surface via EDX is in need of further investigation to explain the overall balance in the calcium to phosphorus atomic ratio. EDX will have to be carried out at the grain boundaries and throughout other features not shown in Figure 2. Longer low-gravity conditions (available on the International Space Station) may also prove to produce a more homogeneous sample. The processing conditions greatly affect the surface chemistry, which is directly related to the bioactivity of the sample in-vivo. The pore formation of the TiB-Ti system is also shown to be controlled by gravity forces. Spherical-like pores are produced in low gravity while longitudinal-radial pores are produced in terrestrial conditions. The latter is predominately due to the deformation force exhibited by gravity. The pore structure is directly related to the strength of the material, in-vivo vascularization, and tissue ingrowth properties. N&Ti and NiTi* were formed together with the equiatomic NiTi during SHS according to the XRD results obtained in Figure 5. The formation of equiatomic NiTi only happens in a narrow almost equiatomic region (50-55 atomic %) in the phase diagram. The NiTi phase exhibits shape memory and superelasticity properties that are desirable for specific implant applications. The NiTi system is continually being investigated to produce greater amounts of the equiatomic NiTi phase and with greater amounts of porosity. The Tic-Ti reaction system has demonstrated the ability of the system to create a FGM via terrestrial gravity conditions. This material produced is functionally graded in porosity and in composition. The upper section posses a -70% apparent porosity and the lower section possesa 30% apparent porosity. The upper section is composed of -100% TiC while the lower section is composed of -50 atomic% Ti and -50 atomic% TiC phases. At the reaction front a large amount of liquid is formed (Ti) and has been gravity-driven downward, thus creating a gravity-driven fluid flow of the liquid Ti to the bottom of the sample. This systemhas the ability to “self-assemble” a material that is graded in specific porosity and composition depending on the initial composition of the reactants. By coupling the Ca@O& system with the NiTi system a “smart implant” could be constructed. The combination of desirable properties could be used to manufacture the “smart implant”, such as the Ca@O& with its resorbable and good surface chemistry properties combined with the superelasticity and shape memory properties of the NiTi system. In caseswhere strength is an issue, the Tic-Ti or TiB-Ti system coupled with the Ca3(P0& system is beneficial. A classof “smart implants” that are functunally graded can also be created through this same method. By combining the known properties of the SHS produced materials a “smart implant” can be constructed providing optimum results in-viva. ACKNOWLEDGEMENTS This work was supported by the NASA Space Product Development Program through the Center for Commercial Applications of Combustion in Space at the Colorado School of Mines under Cooperative Agreement Numbers NCCW-0096 and NCC8-238, and by the NASA Microgravity Research Division under Cooperative Agreement Numbers NCC3-659. Additional funding was provided by the Colorado Commission on Higher Education, the Colorado School of Mines and the CCACS Industrial Partners. REFERNCES Gortcheva, V. P., G. R. Morales, J. J. P. Valdes, et al., Fibrous growth of tricalicum phosphate ceramics,J. ofMat. Sci.: Mat. in Medicine, 8,297-301, 1997. Moore, J. J., An examination of the thermochemistry of combustion synthesisreactions, Processing and Fabrication of Advanced Materials IZI, The Minerals, Metals & Materials Society, 817-831, 1994. Peters,F., M., Schiller C., M. Siedler, et al., Functionally graded materials of biodegradeable polyesters and bonelike calcium phosphates for bone replacement, Functionally Graded Materials 2000, Ceramic Transactions, 114,97-107,2000. E-mail addressof M. Castillo [email protected] Manuscript received 19 October 2002; revised 10 February 2003; accepted 11 March 2003