Ceramic ion exchange: effect in strength and slow crack growth

Objectives: determine the effect of ion exchange in the parameters of slow crack growth – SCG (n and s_f0), Weibull parameters (m and s₀) and construct the Strength–Probability–Time (SPT) diagram of a leucite-reinforced feldspathic ceramic.

Methods: 140 disk-shaped ceramic specimens (12mm in diameter X 1mm-thick) were fabricated according to manufacturer's instructions, polished through 1µm and divided into 2 groups: GC (control) and GI (submitted to an ion exchange procedure using KNO₃ paste at 470° for 15 min). SCG parameters were determined by biaxial flexural strength test in artificial saliva at 37°C using 5 constant stress rates (n=10): 10², 10¹, 10⁰, 10^-1, 10^-2 MPa/s. Additional 20 specimens of each group were tested at 1 MPa/s in the same apparatus to determine Weibull parameters. Using the data obtained in these tests, the SPT diagram was constructed using the least-squares fit of the tensile strength data versus probability of failure.

Results: Mean values of m and s₀ (95% confidence interval) and n and s_f0 (± standard deviations) were: 13.8 (10.0 - 18.8), 60.4 (58.5 - 62.2), 24.1 ± 2.5 and 58.1 ± 0.01 for GC and 7.4 (5.3 - 10.1), 136.8 (129.1 - 144.7), 36.7 ± 7.3, 127.9 ± 0.01 for GI, respectively. Fracture stress (MPa) calculated using the SPT diagram for lifetime of 1 day, 1 and 10 years for 5% failure probability were 31.85, 24.94 and 22.69 for GC and 62.36, 48.82 and 44.37 for GI.

Conclusion: for this porcelain, the ion exchange process resulted in an increase in strength (as represented by s_f0 and s₀) and in the resistance to SCG (n). However, the material's reliability decreased, since there was a decrease in the Weibull modulus. In all lifetimes analyzed, fracture stresses at 5% failure probability were approximately 50% higher for the ion treated group compared to the control.