Mechanical properties and microstructure characteristics of lattice-surfaced PEEK cage fabricated by high-temperature laser powder bed fusion

doi:10.1016/j.jmst.2022.03.009

Abstract

Abstract:

Porous structure design on the contact surface is crucial to promote the osseointegration of the intervertebral cage while preventing subsidence and displacement. However, the stress response will undergo significant changes for the current random porous cages, which can directly affect the mechanical properties and long-term usability. Here, this paper proposed a newly designed polyetheretherketone (PEEK) cage with the triply periodic minimal surface (TPMS)-structured lattice surfaces to provide tailored 3D microporosity and studied the mechanical performance, stress/strain responses, and microstructure changes in depth. The lattice-surfaced PEEK cage mainly exhibits a multiple-point-plane stress transfer mechanism. The compression modulus and elastic limit can be adjusted by controlling the area of the Diamond TPMS surface while the energy absorption efficiency remains stable. The microstructure of high-strength PEEK is featured by the radial pattern morphology. Meanwhile, the double-stranded orthorhombic phase is more ordered, and the benzene plane subunit and lattice volume become more expanded.

Key words: Additive manufacturing, Laser powder bed fusion, Polyetheretherketone, Intervertebral cage, Mechanical properties

Peng Chen, Jin Su, Haoze Wang, Lei Yang, Haosong Cai, Maoyuan Li, Zhaoqing Li, Jie Liu, Shifeng Wen, Yan Zhou, Chunze Yan, Yusheng Shi. Mechanical properties and microstructure characteristics of lattice-surfaced PEEK cage fabricated by high-temperature laser powder bed fusion[J]. J. Mater. Sci. Technol., 2022, 125: 105-117.

Figures/Tables 16

Fig. 1. Powder properties and HT-LPBF processability. (a) Particle size and distribution, and (b) sintering window tested by a heating and cooling DSC cycle. Microscopic morphologies of (c) PEEK powder, (d) sintered monolayer, and (e) sintered bilayer.

Fig. 2. Effects of laser power on (a) ultimate tensile strength, while h is set as a constant of 0.2 mm and V is 2000 mm s-1. (b) Laser sintering condition of PEEK under 35 W when the EMR is 10.64. Effects of laser scanning velocity on (c) ultimate tensile strength, while h is set as a constant of 0.2 mm and P is 15 W. (d) Laser sintering condition at 500 mm s-1 and 15 W when the EMR is 18.24.

Fig. 3. (a) Scatter diagram of the relationship between ultimate tensile strength and EMR, and its GaussAmp fitting curve. (b) Stress-strain curve at the EMR of 15.2.

Fig. 4. Structural diagram of lattice-surfaced PEEK cage. (a) Side view of designed cage placed between vertebral bodies, (b) STL model of PEEK cage with lattice structures designed on the surface and the bottom, (c) Diamond TPMS lattice structure with a relative density of 20% and unit size of 2 mm, and (d) topology of Diamond unit cell.

Fig. 5. Schematic diagram of the intervertebral cage of PEEK. (a) PEEK cage with lattice structures designed on the surface and the bottom. (b) Standardized PEEK cage without lattice surface. (c) Internal Diamond structure of PEEK cage fabricated by HT-LPBF. (d) 3D map of the Diamond structure under the observation of an ultra-depth three-dimensional microscope.

Fig. 6. (a) Compressive stress-strain curve and its 1st derivative curve for the analysis of deformation stages: (1) elastic deformation, (2) yield strengthening, and (3) densification. (b) Comparison of stress-strain curves of cages with different lattice-surfaced areas.

Table 1. Mechanical properties of PEEK cage with different lattice-surfaced areas.

Lattice-surfaced PEEK cage	Compression modulus E_c (MPa)	Yield strength σ_A (MPa)	Elastic limit σ_Pe (MPa)	Energy absorption efficiency (%)
15 × 7 mm²	200.9 ± 24.8	-	33.1 ± 3.2	17.3 ± 0.7
12 × 5 mm²	310.3 ± 18.4	52.9 ± 6.5	43.6 ± 5.0	17.5 ± 0.4
8 × 3 mm²	322.5 ± 26.6	45.7 ± 2.3	47.9 ± 1.9	18.2 ± 0.1

Fig. 7. (a) Comparison of energy absorption efficiency curves, and the illustration is the integral calculation for energy absorption efficiency. (b) Compression modulus and elastic limit of cages with different lattice-surfaced areas.

Fig. 8. Stress distribution at different views and heights of intervertebral cage. (a) Front view, (b) head view, and (c) tail view of the cage. Cross-section views of (d) middle height of the cage, (e) interface between Diamond surface and solid cage, and (f) interior of Diamond surface.

Fig. 9. Stress distribution in a cage slice with the one-period thickness of Diamond. (a) Standardized cage, (b) lattice-surfaced cage, and (c) enlargement of the dotted part of (b).

Fig. 10. Strain distribution of cage slices from different views. X-Z plane views of (a) standardized cage and (b) lattice-surfaced cage. (c) and (d) are Y-Z plane views of the two cages, respectively.

Fig. 11. Cross-sectional morphology characteristics of HT-LPBF processed PEEK at EMR of (a) 6.08, (d) 7.60, (g) 9.12, (j) 10.64. (b, c), (e, f), (h, i), and (k, l) are the enlarged morphologies at the corresponding EMR. z is the vertical building direction, and y is the horizontal printing direction.

Fig. 12. Comparison of (a) DSC curves between PEEK powder and HT-LPBF processed PEEK cages. (b) DSC curves of HT-LPBF processed PEEK cages under different EMR. (c) XRD curves and (d) evolution of lattice parameters of PEEK powder and HT-LPBF processed PEEK cages under different EMRs. (e) Schematic diagram of the change of orthorhombic unit cell and (1 0 0) crystal plane.

Table 2. Comparison of thermal properties between PEEK powder and HT-LPBF processed PEEK under different EMRs.

Empty Cell	Empty Cell	Peak melting temperature $T_{\text{peak}}^{\text{m}}$.(°C)	Melting range Mr (°C)	Melting enthalpy $\text{ }\!\!\Delta\!\!\text{ }{{H}_{m}}$ (J g^–1)	Crystallinity ${{X}_{\text{C}}}\ $(%)	Initial crystallization temperature ${{T}_{\text{ic}}}$ (°C)	Peak crystallization temperature $T_{\text{peak}}^{\text{c}}$ (°C)	Crystallization enthalpy ΔHc (J g^-1)
PEEK powder		338.46	23.31 (324.76-348.07)	32.45	24.96	297.45	293.05	-39.93
HT-LPBF processed PEEK under different EMR	6.08	343	13.69 (334.81-348.5)	12.56	9.66	298.81	292.07	-24.20
	7.60	343.18	11.7 (335.9-347.6)	11.06	8.51	298.82	292.25	-22.61
	9.12	343.35	10.56 (336.57-347.13)	11.38	8.75	299.38	291.25	-25.37
	10.64	344.02	13.23 (334.96-348.19)	13.39	10.3	300.32	292.58	-29.39

Table 3. Crystallography data of PEEK powder and HT-LPBF processed PEEK cages with different EMR.

Empty Cell	EMR	lattice parameter (Å)			lattice volume (Å³)
Empty Cell	EMR	a-axis	b-axis	c-axis	lattice volume (Å³)
PEEK powder	-	7.84	5.97	10.26	480.22
HT-LPBF processed PEEK cage	6.08	7.86	5.96	9.99	467.99
	7.60	7.84	5.95	10.09	470.68
	9.12	7.84	5.97	10.12	473.66
	10.64	7.84	5.95	10.22	476.74

Fig. 13. FT-IR spectra of PEEK powder and HT-LPBF processed PEEK cages with different EMR. FT-IR spectra in the range of (a) 2550-3450 cm-1, (b) 1120-1320 cm-1, (c) 1400-1800 cm-1, and (d) 650-950 cm-1, respectively. Degradation products of (e) dibenzofuran and (f) biphenyl formed by the recombination of adjacent radicals.

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