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Abstract
Ultrafast laser processing of zirconia/alumina nanocomposite ceramics, the current gold standard material for ceramic bearing components in orthopedics, was investigated. Instead of considering the substrate as a homogeneous material, as commonly assumed in laser micromachining, the damage behavior of different phases around the laser ablation threshold upon ultrafast laser irradiation was investigated. Under appropriate experimental conditions, the zirconia phase was selectively ablated while the alumina phase remained intact. The origin of this selective ablation behavior and its relationship with the material band gaps were discussed. Due to the nonlinear absorption mechanisms under ultrafast laser irradiation, the zirconia phase, with its band gap of 5.8 eV, can absorb more laser energy than the alumina phase which has a larger band gap of 8.8 eV. The negligible heat diffusion length ensures that the absorbed laser energy remains confined in the individual phases, leading to the selective ablation of zirconia phase under the given laser fluence. Based on this observation, an ultrafast laser selective phase removal method which can be used to modify the surface composition of nanocomposite materials consisting of phases with different band gaps was proposed.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With a pulse duration shorter than the electron-phonon relaxation time, ultrafast lasers have been extensively used for material processing with high precision as well as minimized thermal effects [1–3]. In ultrafast laser material processing, the laser energy is firstly absorbed by the electrons of the material through different mechanisms that are not only material dependent, but also influenced by the laser intensity [4,5]. In the case of metallic materials, the free electrons in the conduction band directly absorb photons through the inverse Bremsstrahlung process. For materials with band gaps larger than the energy of a single photon, such as semiconductors and dielectrics, the intense laser field of an ultrashort laser pulse can cause strong-field ionization, including multiphoton absorption and tunneling ionization, and can excite the electrons from the valence band to the conduction band [6,7]. The conduction band electrons can then further gain energy by absorbing photons through the inverse Bremsstrahlung process. Those electrons with high kinetic energy can collide with the bound electrons causing impact ionization which is characterized by a multiplication of the free electron density [8,9]. The free electron density is often used as a criterion when modelling the ablation thresholds of dielectrics [10,11]. In general, the materials are treated as homogeneous during theoretical modelling. For materials that are inhomogeneous at the microscale, such as nanocomposite ceramics which are composed of different phases with grain sizes below 100 nm, the free electron density distribution will be non-uniform across different phases upon ultrafast laser irradiation. Therefore, the conventional theoretical approach may no longer be appropriate for describing the material response of a nanocomposite ceramic.
Reported experimental studies on ultrafast laser processing of nanocomposite ceramics mainly focused on applications, such as creating textured surfaces on alumina toughened zirconia for improving the osseointegration process of dental implants [12–16], micromachining of carbon nanotube-metal-oxide nanocomposites which can be used in micro-electro-mechanical systems [17], or structuring of diamond-like nanocomposite films for reduced friction [18]. In those investigations, the applied laser fluences were much higher than the laser ablation threshold of the materials for efficient material removal. Since the grain sizes were much smaller than the laser spot size, the influence of the intrinsic microstructural inhomogeneity of the nanocomposite materials on the machining results was not evident. However, when the applied laser fluence is around the ablation threshold, there will only be a small area in the laser spot center where the local laser fluence is larger than the ablation threshold due to the non-uniform energy distribution characteristic of a Gaussian beam. Under these conditions, the inhomogeneity of the nanocomposite material is expected to have a substantial influence on the laser processing results and should therefore not be overlooked.
An example of such nanocomposite materials are zirconia/alumina ceramics which have found a widespread use as ceramic bearing components in orthopedic joint applications, because of the combination of a high hardness and concomitant wear resistance owing to the presence of alumina and the excellent fracture toughness of zirconia. In this paper, we investigated the material response of zirconia/alumina nanocomposite ceramics under ultrafast laser irradiation at a fluence near the ablation threshold where the material inhomogeneity is not neglectable. Special attention was paid to the influence of the applied laser fluence on the ablation behavior of the different phases. A selective phase removal phenomenon was observed in zirconia/alumina nanocomposite ceramics. Based on this observation, a general method is proposed for the selective removal of specific phases on the surface of a nanocomposite material by ultrafast laser irradiation.
2. Materials and methods
2.1 Ceramic processing
The single-phase zirconia and alumina and zirconia/alumina nanocomposite ceramics were prepared from commercially available starting powders by pressureless sintering. For the composite ceramic, 20 wt% of 3 mol% Y2O3-stabilized ZrO2 (TZ-3Y, Tosoh Corporation, Japan, 50 nm) was wet mixed with 80 wt% Al2O3 (TM-DAR, Taimicron, Taimei Chemicals Co., Ltd., Japan, 0.22 μm) in ethanol for 24 h on a WAB Turbula multidirectional mixer with 5 mm ZrO2 milling media. The ethanol was removed from the mixed powder on a rotating evaporator and oven-dried overnight at 85°C. The dried powder was uniaxially pressed (25 kN, 1 min) and subsequently cold isostatically pressed (300 MPa, 1 min) into a green powder compact, that was sintered at 1500°C in air for 2 h. The heating and cooling rate was set at 10°C/min from 40–1000°C and 5°C/min from 1000–1500°C. For the preparation of single-phase zirconia and alumina ceramics, no mixing procedure was required. The pressing and sintering processes were the same as for the zirconia/alumina composite. The sintered ceramics were first polished with diamond paste (Struers diamond pastes) of different grain sizes in a decreasing order from 15 μm to 3 μm to 1 μm, and then a final step of polishing with a SiO2 polishing suspension to obtain a smooth surface finish. The surface roughness (Sa) was measured with confocal microscopy (S-neox, Sensofar). The large-area surface roughness of both zirconia and the composite were below 10 nm. The scratches on the zirconia surface were polishing marks. The surface defects occurring on the alumina surface, as shown in Fig. 1(c), were due to grain pull-out during polishing which significantly increased the large-area surface roughness, while the small-area surface roughness was still below 10 nm when the defects were excluded. The ceramics were cleaned with acetone for dust removal prior to the laser processing.
Fig. 1. Representative SEM images and surface roughness measurement results of the materials: (a) zirconia; (b) zirconia/alumina nanocomposite with 80% of alumina; (c) alumina. The sub-images in the same column represent the same material.
2.2 Femtosecond laser processing
The laser source for material processing was a femtosecond laser from Amplitude Systems (SATSUMA) with a Gaussian beam profile. The full width at half maximum (FWHM) pulse width was 250 fs with a maximum pulse energy of 20 µJ. The laser wavelength was 1030 nm and the focal length of the optical lens was 100 mm. The total optical loss of the system is 21.5%. Stationary spot machining with single- and multi-pulse as well as line scanning experiments were performed under normal atmospheric conditions. The experimental parameters are listed in Table 1. The laser processed surface was examined by scanning electron microscopy (SEM, XL30 FEG, FEI).
Table 1. Experimental parameters for spot machining and line scanning
3. Results and discussion
3.1 Laser ablation thresholds
The laser ablation threshold is a critical parameter for the laser micromachining process. Various phenomena are expected to happen around the laser ablation threshold, such as material removal and the formation of laser-induced periodic surface structures (LIPSS). A common approach to determine the laser ablation threshold of a material is the diameter extrapolation method [19]. In the single-pulse laser ablation experiment, the diameter of the laser ablation crater (D) was measured as a function of the laser pulse energy (E0). By fitting the data to Eq. (1), the laser beam radius (ω0) at 1/e2 of the peak fluence (F0) was determined by the slope of the fitted line.
where Fth denotes the ablation threshold, and for the Gaussian beam,After determining the beam radius, F0 can be calculated using Eq. (2). Then, the data points can be re-plotted and lines fitted to D2 with respect to ln(F0), and the single-pulse laser ablation threshold can be determined according to the fitted line. The same procedure can be applied to determine the laser ablation threshold under multiple-pulse irradiation. Due to the incubation effect [20], the multi-pulse laser ablation threshold is always smaller than the single-pulse ablation threshold, and their relationship can be described by [21]:
where N is the number of pulses, S is the incubation coefficient. Figure 2 shows the SEM images of the typical laser ablation craters of zirconia and alumina after single- and multiple-pulse irradiation at a laser fluence of 14.8 J/cm2. Not only the depth, but also the diameter of the crater increased with increasing number of pulses due to the incubation effect.Fig. 2. Representative SEM images of the laser ablation spots with 1, 10, 100 pulses at a laser fluence of 14.8 J/cm2 for single-phase (a) zirconia and (b) alumina ceramics.
The experimental results are plotted in Fig. 3. The diameter of the laser focus spot was 16 μm. The single-pulse laser ablation threshold for zirconia and alumina were calculated to be 4.72 and 9.07 J/cm2, respectively. The incubation coefficient was 0.684 for zirconia and 0.555 for alumina.
Fig. 3. (a) Squared diameter of laser ablation craters in single-phase zirconia and alumina ceramics as a function of the logarithm of the laser fluence (J/cm2) under single-pulse laser irradiation. Error bars represent the standard error of four measurements. The calculation of the single pulse laser ablation threshold of alumina is done by assuming that the slope of the fitting line is the same than that of zirconia, which is reasonable since the slope is determined by the laser beam radius which is a constant throughout the experiment. (b) Logarithm of the multi-pulse laser ablation threshold minus logarithm of the single-pulse laser ablation threshold as a function of the logarithm of the number of pulses. Each point in (b) is obtained by the line fitting method as illustrated in (a).
3.2 Selective removal of the zirconia phase
To investigate the material ablation behavior around the laser ablation threshold, a line scanning experiment was performed. The experimental parameters for the line scanning are listed in Table 1. Using Eq. (3), the multi-pulse laser ablation threshold under 16 pulses irradiation was calculated to be 1.97 J/cm2 for zirconia and 2.64 J/cm2 for alumina. Figure 4 shows the representative SEM images of the selected laser scanned area of zirconia/alumina nanocomposite surface under different applied laser fluences. The bright phase is zirconia and the dark phase is alumina. The energy-dispersive X-ray (EDX) point analysis results of the two phases is shown in Fig. 4 (d). When the applied laser fluence was 2.02 J/cm2, which was slightly larger than the ablation threshold of zirconia under 16 pulses laser irradiation but smaller than that of alumina, small laser-induced defects were found to occur on the zirconia phase, preferentially located at the grain boundaries between the zirconia and alumina phases as shown in Fig. 4(b). This is because of the crystal lattice discontinuity, most frequently accompanied by atomic segregation and the formation of a space charge layer in oxide ceramics along the grain boundaries after sintering. As indicated by the intergranular fracture mode in these composites, the bond strength across the grain boundaries is weaker than in the bulk of the grains. At an applied laser fluence of 2.24 J/cm2, which was much larger than the ablation threshold of zirconia under 16 pulses laser irradiation but still smaller than that of alumina, much larger laser-induced defects were observed, mainly in the zirconia phase as shown in Fig. 4(c). By comparing Fig. 4(c) with Fig. 4(a), which was taken before laser ablation, it can be proved that at least part of the zirconia phase was removed and cavities formed on the laser irradiated surface, while the alumina phase remained nearly intact, as evidenced by the retaining of a smooth and flat surface of the alumina phase region. The cavity formation might be able to induce local enhancement of the electric field due to the excitation of surface plasmons which could further enhance the local laser absorption [22,23]. The plasmon generation, which normally happens in metals, could also occur in dielectrics upon ultrafast laser irradiation by exciting the electrons from the valence band to the conduction band transiently turning the dielectric surface into a metallic state [24]. When further increasing the applied laser fluence to exceed the ablation thresholds of both phases, as shown in Fig. 4(e), the original flat surface was totally destroyed, resulting in a rough surface.
Fig. 4. Representative SEM images of different degrees of zirconia phase selective ablation for zirconia/alumina nanocomposite ceramics at different applied laser fluences: (a) SEM image before laser ablation; (b) slight ablation of the zirconia phase at laser fluence 2.02 J/cm2; (c) selective removal of the zirconia phase at laser fluence 2.24 J/cm2. It can be seen that the shape of the ablated cavity in the center of the image coincides with the shape of the corresponding zirconia phase before ablation. (d) EDX analysis of spot 1 and 2 in (c); (e) severe ablation of both phases at laser fluence 2.96 J/cm2. Alumina is the dark phase, zirconia is the bright phase.
3.3 Ultrafast laser selective phase removal mechanism
In ultrafast laser processing of dielectrics, free electron generation occurs through impact ionization, multiphoton absorption and tunneling ionization, which are highly non-linear processes [4]. For materials with limited intrinsic defects, such as inside the individual grains of the nanocomposite ceramic, the initial density of free electrons is low. The strong-field ionization process will provide seed electrons for impact ionization. The strong-field excitation rate can be calculated by Keldysh’s model which reveals itself as a rather complicated expression associated with the Keldysh parameter, γ, being $\gamma = \omega \sqrt {mU} /({eE} )$, in which ω is the angular frequency of the laser light, e and m are the electron charge and reduced mass, U is the ionization potential, and E is the amplitude of the electric field of the laser light [6,25]. When γ≫1, strong-field excitation is dominated by multiphoton absorption; while when γ≪1, tunneling ionization will take over as the main excitation mechanism. To understand the role of the material band gap in the strong-field excitation process intuitively, we consider a simplified picture of the multiphoton absorption process. The minimum number of photons, n, that is required to excite a single electron from the valence band to the conduction band is determined by nhν ≥ U, where h is the Planck constant, ν is the photon frequency, and U is the corrected band gap, which is influenced by the laser intensity and is larger than the intrinsic band gap of the material. A larger band gap corresponds to a larger value of n, which implies that the excitation of the valance electrons will be more difficult. As a result, the material will be more resistant to laser ablation. The ablation threshold of a dielectric material is reported to be linked to its band gap by a power law with an exponent varying from 2.5 to 3 [26–28]. The band gaps of zirconia and alumina are around 5.8 eV and 8.8 eV, respectively [29,30]. With a larger band gap, alumina is therefore expected to be more resistant to laser ablation compared to zirconia. This implies that under certain laser processing parameters, the zirconia phase could be ablated and removed while the alumina phase remained intact, which was confirmed by our experiments as shown in Fig. 4. The non-linear effect of the laser energy absorption of dielectrics features a unique threshold fluence for each phase below which no ablation would occur.
Taking advantage of this phenomenon, it is possible to develop a new surface treatment method that can selectively remove certain phases from the surface of a nanocomposite material using an ultrafast laser. Figure 5 shows a schematic diagram of the mechanism of ultrafast laser selective removal of the zirconia phase from the zirconia/alumina nanocomposite surface.
Fig. 5. Schematic diagram for the ultrafast laser selective removal of the zirconia phase from the surface of a zirconia/alumina nanocomposite ceramic. Alumina has a larger band gap than zirconia. This suggests that the laser ablation threshold of alumina is higher than for zirconia due to the non-linear absorption mechanism under ultrafast laser irradiation. When a laser fluence between the ablation thresholds of the two phases is applied, the zirconia phase can be selectively removed without damaging the alumina phase. The sub-figure that shows the relationship between band gap and ablation threshold is reproduced with permission from [26]. Copyright ©2009 American Physical Society.
A large band gap contrast between the different phases, which corresponds to a large difference of their laser ablation thresholds, is supposed to be beneficial for selective phase removal. Another key factor that makes the selective phase removal method possible is that the laser pulse energy deposition and subsequent material ablation occur in a very short timescale during which heat diffusion can be neglected. The pulse duration of an ultrafast laser can be shorter than the electron-phonon relaxation time which is around several picoseconds. During the interaction of an ultrafast laser and dielectrics, the laser energy will firstly be absorbed by electrons through non-linear absorption processes and then transferred to the lattice through electron-phonon coupling, leading to a temperature increase of the material and eventually material ablation. It takes several picoseconds to reach electron-phonon equilibrium and heat the lattice, while the material ablation starts from a time scale in the order of magnitude of 100 picoseconds [31]. At this time scale (t), the characteristic heat diffusion length (μ) in the alumina phase can be estimated to be $\mu = 2\sqrt {\kappa t/({\rho c} )} $, where κ, ρ, and c are thermal conductivity, density, and specific heat capacity of alumina, respectively [32]. Taking the temperature dependent thermal properties of alumina into account [33], when the temperature of alumina phase is relatively high due to laser heating, for example between 1000–2000°C, the heat diffusion length is around 20–23 nm. This value is considerably small compared to the average grain size of alumina which is of the order of 1 µm. When the thermal boundary resistance is considered [34], the heat flux from the zirconia to the alumina phase is further reduced. When considering single-pulse laser ablation, a negligibly small heat diffusion length, together with a reduced heat flux, ensures that the laser energy being absorbed by the zirconia phase will be able to confine in the individual grains before ablation happens. When ablation occurs, most of the absorbed laser energy will be carried away by the ablated material. A small portion of the absorbed laser energy will then dissipate into the bulk material leading to the temperature increase of the surrounding area. Since zirconia has a smaller band gap than alumina, the zirconia phase has a larger strong-field ionization rate and therefore can absorb more energy than alumina under the same laser fluence. Together with a smaller volumetric heat capacity, the temperature of the zirconia phase can be much higher than that of alumina after laser irradiation. Therefore, ablation can initiate from the zirconia phase, as is shown in Fig. 4(b). For multi-pulse laser ablation situations, this process will be repeated for each pulse, and this may lead to a temperature increase of the surrounding area due to a heat accumulation effect. Yet, since most of the absorbed laser energy is carried away by the ablated materials, the heat affected zone will be minimized, and the impact to the surrounding alumina grains could be largely avoided.
It is interesting to note that the melting point of zirconia is 2715°C, while it is 2072°C for alumina [35]. This indicates that the temperature heterogeneity of the ceramic composite is huge under ultrafast laser irradiation: in the zirconia phase region, the temperature will be higher than 2715°C for sufficient material ablation, while it needs to be lower than 2072°C for the alumina phase region to ensure it can remain intact. This is only possible when the heat diffusion is a much slower process compared to the timescale for the laser energy deposition and lattice heating, which is exactly the case for ultrafast laser irradiation. The fact that the zirconia phase, which has a higher melting point, was removed leaving the surrounding alumina phase intact suggests the subsequent material ablation took place before thermal equilibrium was established between the zirconia and alumina phases.
3.4 Potential applications and generalization of the selective phase removal method
Unlike many other surface treatment methods, such as sandblasting, non-selective chemical etching, laser surface texturing, and traditional mechanical machining, for which modifying the surface topography is the main goal, the ultrafast laser selective phase removal method presented here is aimed at modifying the surface composition of a nanocomposite material composed of different phases. Following the removal of specific phases, the surface topography will inevitably be altered. This may be either an added value or an adverse effect to the final product depending on the desired functionality. Potential applications can be found where the requirement for the surface composition is different from the bulk material to fulfill certain functions. The benefits of this method can be two-fold. Firstly, in analogy to selective chemical etching [36], which is using a chemical etchant to dissolve unwanted chemical elements or compounds, this method offers a novel physical way of modifying the surface composition of a composite material which does not need to introduce new chemicals in the environment. Secondly, this method is characterized by the removal of individual grains, leaving small cavities formed in the treated surface. The size of the cavities is determined by the grain size which can be much smaller than the laser beam diameter. Therefore, this method can also be used to modify the surface topography of a composite material with a much smaller feature size compared to the conventional laser surface texturing process in which the feature size of the texture is usually limited by the laser spot size. This might be helpful in improving the wear performance of the treated surfaces by trapping of lubricant in some applications where dramatic change of the original surface is unfavored.
In general, this ultrafast laser selective phase removal method can also be applied to a nanocomposite material composed of more than two phases. A generalized concept of this method is shown in Fig. 6 in which a four-phase composite is taken as an example. For simplicity, we assume that the laser ablation threshold of a phase is exclusively determined by its band gap. Then, for a nanocomposite material composed of four phases with different band gaps, each phase is characterized by a unique laser ablation threshold. When the material is subject to laser irradiation, we assume that the phases with a laser ablation threshold smaller than the applied laser fluence will be removed completely, while the other phases will remain intact. By applying an appropriate laser fluence F, selective removal of specific phases can be realized.
Fig. 6. Schematic diagram of the generalized ultrafast laser selective phase removal method. For a material composed of four phases with different band gaps, and thus different laser ablation threshold values, when an appropriate laser fluence is applied, the phases with ablation thresholds smaller than the applied laser fluence can be removed selectively, while the other phases remain intact.
Finally, we want to address the issues regarding the determination of the ablation thresholds of the materials in this research. Firstly, the incubation coefficient is only a fitted value based on a phenomenological model which does not take the saturation effect into consideration [37]. The prediction often deviates from experimental result under a large number of applied laser pulses and a new ablation threshold needs to be calculated taking into account the saturation effect. Moreover, the laser ablation threshold is also influenced by the laser spot size. A larger spot size normally leads to a smaller ablation threshold due to the presence of material defects [38]. In the selective phase removal process, the individual phases were much smaller than the laser spot size. Therefore, the removal of individual phases may require a higher laser fluence than the ablation threshold obtained with the bulk material. Secondly, in the line scanning experiment, the laser spot moved continuously. Whether the equivalent number of applied laser pulses can be compared to the stationary spot machining situation is questionable.
Besides the diameter regression method that was adopted in this research, there are other methods of determining the laser ablation threshold of a material, including laser beam deflection method [39], direct optical observation [40], volume regression method [41], laser-induced photoemission [42], etc. Different measurement methods can give results with large variation even under the simplest single-pulse laser ablation condition in which the incubation effect is absent [43]. Therefore, a precise prediction of the multi-pulse laser ablation threshold is difficult. In addition, the selective phase removal behavior is sensitive to the applied laser fluence, while there is always some level of intensity fluctuation in a laser beam, both temporally and spatially. This will complicate the attempt to determine the correlation between the applied laser fluence and the laser ablation threshold of the material in practice. Despite these limitations, the main conclusion of the presented research, which is the demonstration of the principle of selectively removing certain phases from a multi-phase material with an ultrafast laser, is still preserved. Further research is needed to get a deeper understanding of the selective phase removal mechanism which will then be helpful for the implementation of this method in practical applications.
4. Conclusions
In summary, we investigated the interaction of an ultrafast laser with a zirconia/alumina nanocomposite ceramic incorporating two phases having different band gaps. A selective phase ablation phenomenon was observed when the applied laser fluence was around the ablation threshold. Ablation was found to occur exclusively in the zirconia phase, while the alumina phase remained intact. The ablation mainly initiated at the zirconia-alumina grain boundaries and propagated to the zirconia phase leading to the selective removal of the affected material. Based on this observation, an ultrafast laser selective phase removal method was proposed. The role of the band gaps of the constituent phases as well as the influence of thermal effects were discussed. Two phenomena were identified as key enablers for the selective phase removal process. Firstly, the nonlinear laser absorption mechanism of wide band gap materials under strong laser field ensures that the amount of laser energy absorption by different phases of the nanocomposite materials is determined mainly by the band gaps of the materials; a smaller band gap will lead to more laser energy absorption and therefore, the corresponding phase will be ablated first regardless of the melting points of the phases. Secondly, ultrafast laser irradiation can deposit a high amount of energy in an ultrashort timescale so that thermal diffusion can be neglected. Because of this, the temperature heterogeneity between different phases can be huge enough to cause the higher melting point phase to melt first while the lower melting point phase remains intact.
The performance of the selective removal method may not be solely determined by the ablation threshold differences of the phases. The incubation effect at the microscopic scale, the grain sizes as well as the thermal properties of different phases, and the presence of defects at the grain boundaries may also play an important role in the process outcome. These are some potential directions for future research. This paper is aimed at presenting a new tool for the surface composition modification of nanocomposite materials which we believe will find technological applications in the future.
Funding
Fonds Wetenschappelijk Onderzoek (G095920N); KU Leuven (C3/20/084, STG-18-00308).
Acknowledgments
The authors thank Eddy Kunnen at SIRRIS for his assistance with the femtosecond laser processing experiments.
Disclosures
The authors declare no conflicts of interest.
Author contributions. Sylvie Castagne and Jide Han designed the experiments; Jide Han and Olivier Malek performed the experiments; Sylvie Castagne, Annabel Braem, Jozef Vleugels and Jide Han contributed to the interpretation of the results; Jide Han prepared the original draft; all authors edited the draft and contributed to the final manuscript.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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