Crack Detection in High-Velocity Oxygen-Fuel-Sprayed Al_59.2Cu_25.5Fe_12.3B₃ Quasicrystalline Coatings Using Piezoelectric Active Sensors

Abstract

Quasicrystals have received some attention in recent years regarding their potential usefulness as low friction and wear resistant materials. When quasicrystalline (QC) coatings are exposed to different levels of external loading or are subjected to thermal stresses, crack nucleation, and propagation become life-limiting factors. In this investigation, Al_59.2Cu_25.5Fe_12.3B₃ QC coatings were deposited onto AISI-A36 steel substrates using the high-velocity oxygen-fuel (HVOF) technique. The coatings were deposited at different oxygen-to-fuel ratios. To characterize the integrity of the coatings, the active piezoceramic excitation technique was applied to detect the propagation of cracks during three-point bending tests. The piezoelectric transducer signal was processed using wavelet transformation analysis. The results revealed that the coated samples displayed different densities of cracks depending on the oxygen-to-fuel. The crack density was found higher for coatings formed using a slightly oxidizing flame. The opening of the pre-existing cracks in the coating was the main reason of signal attenuation during piezoelectric excitation. The HVOF-sprayed AlCuFeB quasicrystalline coating, with thickness between 150 and 180 μm, withstands a flexural stress as high as 340 MPa. To the best of our knowledge, this study is the first that provides a quantitative assessment of the viability of QC coatings in mechanical applications in spite of their intrinsic brittleness.

Appendix: Calculation of the Coating Flexural Stress

The tensile stress in the outer fiber of the coating is calculated based on classical composite beams theory. This model uses the transformed section method to consider the presence of two materials with different elastic modulus. In order to determine stress, both materials were represented as a single solid to calculate neutral axis location and moment of inertia. This transformation is made using the ratio of the elastic modulus:

$$n = \frac{{E_{\text{qc}} }}{{E_{\text{s}} }}$$

(1)

In this case, quasicrystalline material is transformed into steel, as shown in Fig. 12, where: E_qc is the modulus of elasticity of the quasicrystalline coating and E_s is the modulus of elasticity of steel. To find the neutral axis of the equivalent steel beam, the width of the steel is multiplied by n, Eq 2.

$$W_{{ 2 {\text{s}}}} = \frac{{E_{\text{qc}} }}{{E_{\text{s}} }} \cdot W_{\text{qc}}$$

(2)

W_2s and W_qc are the width of the transformed steel and of the coating (15 mm), respectively. The normal bending stress is given by the classical Eq 3.

$$\sigma = \frac{{M\bar{Y}}}{{I_{\text{transformed}} }}$$

(3)

However, the use of the new geometry resulting from transforming the width of the coating is needed. Thus, the centroid $\bar{Y}$ and the transformed moment of inertia of the steel sections are given by Eq 4 and 5, respectively.

$$\bar{Y} = \frac{{\left( {t_{\text{qc}} /2} \right) \times \left( {t_{\text{qc}} \cdot w_{\text{s}} } \right) + \left( {t_{\text{s}} /2 + t_{\text{qc}} } \right) \times \left( {t_{\text{s}} \cdot w_{2} } \right)}}{{\left( {t_{\text{qc}} \cdot w_{\text{s}} } \right) + \left( {t_{\text{s}} \cdot w_{2} } \right)}}$$

(4)

$$I_{\text{transformed}} = \left[ {\frac{1}{12}w_{\text{s}} \cdot t_{\text{qc}}^{3} + w_{\text{s}} \cdot t_{\text{qc}} \left( {\bar{y} - \frac{{t_{\text{qc}} }}{2}} \right)^{2} } \right] + \left[ {\frac{1}{12}w_{2} t_{\text{s}}^{3} + w_{2} \cdot t_{\text{s}} \left( {\frac{{t_{\text{s}} }}{2} + t_{\text{qc}} - \bar{y}} \right)^{2} } \right]$$

(5)

where t_qc is the quasicrystalline coating thickness; t_s is the steel substrate thickness and w₂ is the transformed steel width given Eq 2. The geometric data used to calculate the stress are listed in Table 2.

Fig. 12

New cross section of the beam