Laser Spallation Technique Research Articles

Nickel–silicon interfacial adhesion strength measured by laser spallation

Thin films of amorphous silicon (a-Si) coated on metals such as nickel (Ni) are one of the most promising anode architectures for high-energy-density lithium-ion (Li-ion) batteries. The performance and longevity of batteries with this type of electrode depend on the integrity of the Ni/a–Si interface. The integrity of the a-Si /Ni bonded interface during cycling is critical, but the experimental characterization of interfacial failure of this material system is highly challenging and there is a sparsity of interface strength data in the literature. Here, we describe a laser spallation (LS) technique to characterize the interfacial adhesion strength of Ni/a–Si multilayer films created by chemical vapor deposition (CVD). The LS technique enables the non-contact measurement of the tensile interfacial strength with high precision when compared to conventional methods for characterizing adhesion. Interferometric measurement combined with finite element analysis shows that the Ni/a–Si interface, created via the CVD of a-Si on Ni surfaces can withstand ≈46–72 MPa in tension before failure initiation. To ensure successful and precise characterization of interfacial adhesion strength using LS, we further develop a design criterion for multi-layer samples by analyzing the thin-film mechanics. Our study provides insights into the strength of the Ni/a–Si interface that governs the performance and durability of high-energy-density anodes and offers design guidelines for improving thin-film electrode integrity.

Sucrose-mediated formation and adhesion strength of Streptococcus mutans biofilms on titanium

Biofilms consist of bacterial cells surrounded by a matrix of extracellular polymeric substance (EPS), which protects the colony from many countermeasures, including antibiotic treatments. Growth and formation of bacterial biofilms are affected by nutrients available in the environment. In the oral cavity, the presence of sucrose affects the growth of Streptococcus mutans that produce acids that erode enamel and form dental caries. Biofilm formation on dental implants commonly leads to severe infections and can restrict osseointegration necessary for the implant to be successful. This work determines the effect of sucrose concentration on biofilm EPS formation and adhesion of Streptococcus mutans, a common oral colonizer, to titanium substrates simulating common dental implants. Biofilm formation and profiles are visualized at high magnification with scanning electron microscopy (SEM). Large mounds and complex structures consisting of bacterial cells and EPS can be seen in biofilms at sucrose concentrations that are favorable for biofilm growth. The laser spallation technique is used to apply stress wave loading to the biofilm, causing the biofilm to delaminate at a critical tensile stress threshold. The critical tensile stress threshold is the adhesion strength. Because laser spallation applies the stress loading to the rear of the substrate, bulk adhesion properties of the biofilm can be determined despite the heterogenous composition and low cohesion strength of the biofilm. Statistical analysis reveals that adhesion strength of biofilms initially increase with increasing sucrose concentration and then decrease as sucrose concentration continues to increase. The adhesion strength of bacterial biofilms to the substrate in this study is compared to the adhesion of osteoblast-like cells to the same substrates published previously. When sucrose is present in the biofilm growth environment, S. mutans adhesion is higher than that of the osteoblast-like cells. Results of this study suggest sucrose-mediated S. mutans biofilms may outcompete osteoblasts in terms of adhesion during osseointegration, which could explain higher rates of peri-implant disease associated with high sugar diets. Further studies demonstrating adhesion differentials between biofilms and cells including co-cultures are needed and motivated by the present work.

Measuring adhesion strength of epoxy-based adhesives under quasi-static and dynamic loading conditions

Adhesively bonded joints are very common in aerospace applications. They are used in the fabrication of metal and composite components as well as in their repair work. Besides subjecting the interface to more uniform stress distribution, the bonded joints are simple and more efficient when compared to mechanical fastening. Since the reliability of components and structures depends upon the strength of bonded joints, adhesion strength has become an essential design and fabrication parameter. In this investigation, the interface strength between the metallic base and epoxy-based adhesive is assessed by performing quasi-static and extreme dynamic experiments. New test methods are developed by performing computational analysis with the objective of nullifying the stress concentration effect near the interface edge. Quasi-static adhesion tests performed using modified butt-joint specimens exhibit the interface strength between the aluminum and adhesive as 3.07 MPa. Upon subjecting the interface to a strain rate of ∼107/s by employing laser-induced stress waves, the interface strength is evaluated to be 213 MPa. The laser spallation technique, combined with the Michelson interferometer, was used to perform the optical measurements.

Evaluation of interlaminar adhesion strength of carbon fiber reinforced plastics with different molding pressure and fiber volume fraction by a laser spallation technique

Evaluation of interlaminar adhesion strength of carbon fiber reinforced plastics with different molding pressure and fiber volume fraction by a laser spallation technique

Measurement of adhesion quality of oxide scale on a low-carbon steel with Cu addition with different heat-treatment with a laser spallation technique

Measurement of adhesion quality of oxide scale on a low-carbon steel with Cu addition with different heat-treatment with a laser spallation technique

Induction and amplification of elastic wave into cementitious material by applied laser ablation technique

To identify the elastic wave excitation behavior by means of laser pulse irradiation on cementitious material surface in this study, the application of pulsed laser irradiation system for laser ablation and spallation technique can be explained and proposed for a method for vibration (elastic wave) excitation used in NDT techniques. Expecting laser-induced-plasma-assisted ablation (LIPAA), laser ablation mechanism based on detecting AE (acoustic emission) signal is clarified as well as vibration amplitude and energy amplification by LIPAA with respect to such cementitious materials as cement paste. Elastic wave excitation into cementitious material due to laser ablation is not only influenced by pulse irradiation condition but also laser-induced-plasma-assisted ablation (LIPAA) process. The proposed technique for LIPAA works to induce and amplify the vibration (energy) generation when pulsed laser irradiation is implemented with predetermined fluence on cementitious material.

Measurement of adhesion quality at room temperature and 400℃ of oxide scale on a low-carbon steel with Mn addition with a laser spallation technique

The oxide scale formed on low carbon steel during hot rolling process is removed with high pressure water jets in its production process. Some researchers reported that the adhesive strength of the oxide scale was depended on structure of the scale and chemical composition in substrate. However, those quantitative adhesive strengths are still unclear. This study aims to quantitatively estimate the effect of Mn content and heat treatment processes on adhesive strength of the oxide scale on carbon steel with a laser spallation technique. Maximum estimated adhesive strength for the samples in this study was about 50 MPa. The adhesive strength tended to decrease with increasing Mn content. At temperature of 400℃, the strength for the samples without the structure change decreased by 10 MPa. The strength for some samples at 400 ℃ showed higher due to change in scale structure. In all samples, it was confirmed that the strength was depended on the structure of the scale.

Evolution of the Laser-Induced Spallation Technique in Film Adhesion Measurement.

Laser-induced spallation is a process in which a stress wave generated from a rapid, high-energy laser pulse initiates the ejection of surface material opposite the surface of laser impingement. Through knowledge of the stress-wave amplitude that causes film separation, the adhesion and interfacial properties of a film-on-substrate system are determined. Some advantages of the laser spallation technique are the noncontact loading, development of large stresses (on the order of GPa), and high strain rates, up to 108/s. The applicability to both relatively thick films, tens of microns, and thin films, tens of nm, make it a unique technique for a wide range of materials and applications. This review combines the available knowledge and experience in laser spallation, as a state-of-the-art measurement tool, in a comprehensive pedagogical publication for the first time. An historical review of adhesion measurement by the laser-induced spallation technique, from its inception in the 1970s through the present day, is provided. An overview of the technique together with the physics governing the laser-induced spallation process, including functions of the absorbing and confining materials, are also discussed. Special attention is given to applications of laser spallation as an adhesion quantification technique in metals, polymers, composites, ceramics, and biological films. A compendium of available experimental parameters is provided that summarizes key laser spallation experiments across these thin-film materials. This review concludes with a future outlook for the laser spallation technique, which approaches its semicentennial anniversary.

Biofilm and cell adhesion strength on dental implant surfaces via the laser spallation technique

ObjectiveThe aims of this study are to quantify the adhesion strength differential between an oral bacterial biofilm and an osteoblast-like cell monolayer to a dental implant-simulant surface and develop a metric that quantifies the biocompatible effect of implant surfaces on bacterial and cell adhesion. MethodsHigh-amplitude short-duration stress waves generated by laser pulse absorption are used to spall bacteria and cells from titanium substrates. By carefully controlling laser fluence and calibration of laser fluence with applied stress, the adhesion difference between Streptococcus mutans biofilms and MG 63 osteoblast-like cell monolayers on smooth and rough titanium substrates is obtained. The ratio of cell adhesion strength to biofilm adhesion strength (i.e., Adhesion Index) is determined as a nondimensionalized parameter for biocompatibility assessment. ResultsAdhesion strength of 143 MPa, with a 95% C.I. (114, 176), is measured for MG 63 cells on smooth titanium and 292 MPa, with a 95% C.I. (267, 306), on roughened titanium. Adhesion strength for S. mutans on smooth titanium is 320 MPa, with a 95% C.I. (304, 333), and remained relatively constant at 332 MPa, with a 95% C.I. (324, 343), on roughened titanium. The calculated Adhesion Index for smooth titanium is 0.451, with a 95% C.I. (0.267, 0.622), which increased to 0.876, with a 95% C.I. (0.780, 0.932), on roughened titanium. SignificanceThe laser spallation technique provides a platform to examine the tradeoffs of adhesion modulators on both biofilm and cell adhesion. This tradeoff is characterized by the Adhesion Index, which is proposed to aid biocompatibility screening and could help improve implantation outcomes. The Adhesion Index is implemented to determine surface factors that promote favorable adhesion of cells greater than biofilms. Here, an Adhesion Index ≫ 1 suggests favorable biocompatibility.

The effect of surface roughness on laser-induced stress wave propagation.

We investigate laser-induced acoustic wave propagation through smooth and roughened titanium-coated glass substrates. Acoustic waves are generated in a controlled manner via the laser spallation technique. Surface displacements are measured during stress wave loading by the alignment of a Michelson-type interferometer. A reflective coverslip panel facilitates capture of surface displacements during loading of as-received smooth and roughened specimens. Through interferometric experiments, we extract the substrate stress profile at each laser fluence (energy per area). The shape and amplitude of the substrate stress profile are analyzed at each laser fluence. Peak substrate stress is averaged and compared between smooth specimens with the reflective panel and rough specimens with the reflective panel. The reflective panel is necessary because the surface roughness of the rough specimens precludes in situ interferometry. Through these experiments, we determine that the surface roughness employed has no significant effect on substrate stress propagation and smooth substrates are an appropriate surrogate to determine stress wave loading amplitude of roughened surfaces less than 1.2 μm average roughness (Ra). No significant difference was observed when comparing the average peak amplitude and loading slope in the stress wave profile for the smooth and rough configurations at each fluence.

Biofilm rupture by laser-induced stress waves increases with loading amplitude, independent of location.

Integral to the production of safe and biocompatible medical devices is to determine the interfacial properties that affect or control strong biofilm adhesion. The laser spallation technique has recently emerged as an advantageous method to quantify biofilm adhesion across candidate biomedical surfaces. However, there is a possibility that membrane tension is a factor that contributes to the stress required to separate biofilm and substrate. In that case, the stress amplitude, controlled by laser fluence, that initiates biofilm rupture would vary systematically with location on the biofilm. Film rupture, also known as spallation, occurs when film material is ejected during stress wave loading. In order to determine effects of membrane tension on the laser spallation process, we present a protocol that measures spall size with increasing laser fluence (variable fluence) and with respect to distance from the biofilm centroid (iso-fluence). Streptococcus mutans biofilms on titanium substrates serve as our model system. A total of 185 biofilm loading locations are analyzed in this study. We demonstrate that biofilm spall size increases monotonically with laser fluence and apply our procedure to failure of non-biological films. In iso-fluence experiments, no correlation is found between biofilm spall size and loading location, thus providing evidence that membrane tension does not play a dominant role in biofilm adhesion measurements. We recommend our procedure as a straightforward method to determine membrane effects in the measurement of adhesion of biological films on substrate surfaces via the laser spallation technique.

Adhesion of biofilms on titanium measured by laser-induced spallation.

Eradication of established implant-associated and bacterial biofilm-forming infections remains difficult in part because these biofilms remain well-adhered to the implant surface. Few experimental techniques are available to measure macro-scale strength of bacterial biofilm-implant adhesion. We have adapted the laser spallation technique to compare the macro-scale adhesion strength of biofilms formed on titanium. By using a rapid pressure wave (35 ns) to load the interface, we prevent disturbance of the biofilm surface prior to measurement, and preclude the time necessary for the biofilm to respond to and adapt under loading. Biofilms of Streptococcus mutans, a Gram-positive bacterium associated with human dental caries (cavities) were cultured directly on commercially pure titanium within our custom substrate assembly. Growth conditions were varied by adding sucrose to the Todd Hewitt Yeast (THY) broth: THY control, 37.5 mM, 75 mM, 375 mM, and 750 mM sucrose. Multiple locations on each biofilm were loaded using the laser spallation technique. Loading pressure wave amplitude was controlled by adjusting laser fluence, energy per area. Initially, addition of sucrose to the media increased biofilm adhesion to titanium. However, once a saturation concentration of 75 mM sucrose was reached, increasing the sucrose concentration further resulted in a decrease in biofilm adhesion. This study is the first demonstration of the adaptation of the laser spallation technique to measure bacterial biofilm adhesion. Establishment of this macro-scale biofilm adhesion measurement technique opens the door for many biofilm-surface adhesion studies. We anticipate further work in this area towards understanding the complex relationships among bacteria species, environmental factors, surface characteristics, and biofilm adhesion strength.

Evaluation of residual stress and adhesion of Ti and TiN PVD films by laser spallation technique

The laser spallation technique was applied for measurement of residual stress and adhesion of thin films. Two films of different properties, ductile and soft Ti, and hard and brittle TiN, were studied. The films were produced on 304 steel substrate by PVD method. The residual stress value obtained by laser spallation technique LST were compared with stress value from X-ray diffraction method. Good agreement of stress values measured by both methods was attained. Additionally, the interface strength of the films was tested by laser adhesion spallation technique LASAT with use of VISAR system. It was shown that shock wave induced by a nanosecond laser pulse adequately determines properties of PVD thin films on metal substrate.

Measurement of bond strength of oxide scale with different structure on carbon steel by a laser spallation technique.

Measurement of bond strength of oxide scale with different structure on carbon steel by a laser spallation technique.

Vertically oriented ZrO2[sbnd]TiO2[sbnd]Nb2O5[sbnd]Al2O3 mixed nanopatterned bioceramics on Ti6Al7Nb implant assessed by laser spallation technique

Recently, reports on the fabrication of nonstructural configuration have generated scientific interest due to increased awareness of the oxide nanotubes in the biomedical implant field and other industrial approaches. Here, highly ordered ZrO2TiO2Nb2O5Al2O3 nanotube arrays were grown by physical vapor deposition (PVD) of a zirconium layer on titanium-niobium alloy implant (IMPTi67) and succeeding anodic oxidation at a constant voltage (60 V) with exposure times ranging from 30 to 300 min. To crystallize the resulting mixed oxide nanotubes, heat treatment was also applied. The nanotubes were placed in an atmospheric furnace at temperatures ranging from 450 to 800 °C for 2 h. This treatment provided a morphological evolution was strongly influenced by the anodization time, where highly aligned bamboo-shaped oxide nanotubes (161 ± 44 nm in inside diameter and ∼1 μm in length) were formed after 300 min of anodization. After annealing at temperature below 500 °C, the nano-array configuration remained stable and the average inner diameter decreased to around 40 nm. The effects of anodically fabrication time on the adhesion strength of four-phase multilayered mixed oxide nanotubes on anodized Zr/IMPTi67 surfaces assed by microscratch analysis. From the results, increasing the annealing temperature further to 800 °C fully destroyed the nanotubes and consequently the tubular arrangement was converted to a coarse grain structure. It was revealed that annealing at 450 °C for 2 h was the optimum conditions for stable nanotubular array generation, where the highest adhesion strength (shifted from 2595.59 to 2640.12 mN), microhardness (372 HV) and hydrophilicity (15 ± 1°) were recorded. It is notify that the adhesion strength of highly ordered mixed nanotubes was remarkably decreased after thermal treatment at 800 °C (1817.51 mN). Through the laser spallation analysis, the intrinsic adhesion strength of the Zr/IMPTi67 interface was found to be at 63 MPa, while the first stages of failure occurred at a stress state of 35 MPa. The adhesion of the film with the tubular structure proved to have a significantly larger adhesion strength of 246 MPa for the complete failure and 203 MPa for incipient stages of spallation. The considered surface modification can be an effective step in determining the interface strength between biomedical implants and the nanostructured coatings.

レーザー誘起超音波を用いたメッキ膜の界面強度評価と剥離検出法

This study aims to evaluate interfacial strength of surface coating by using laser induced ultrasonic wave. In this method, pulse laser beam is irradiated on the back surface of substrate, inducing strong elastic wave which propagates through the specimen thickness, and reflects at the top surface of the coating. The wave propagation and reflection generates an expansion wave (tensile stress), leading to the interfacial fracture (i.e. delamination). This is called laser spallation technique. This study changes the energy level of laser irradiation variously, and measures the coating delamination in-situ based on the out-of-plane displacement waveforms. The delamination can be detected by using correlation coefficient calculated with a series of the waveform obtained at epicenter of laser irradiation. The delamination can also be visualized by using laser ultrasonic wave scanning. In parallel, the computation of elastic wave is carried out using FDTD (finite difference time domain) and the interfacial strength is estimated quantitatively.

レーザスポレーション法を用いた低炭素鋼における酸化スケールの高温密着性状評価

This study aims to evaluate adhesion quality of oxide scale on low carbon steel plate at high temperature. The scratch test, indentation test and pull-out test are generally adopted to evaluate the adhesion quality of thin films. However, these methods requires contact between a probe and a specimen, and are not allowed to perform at high temperature. Since a laser spallation technique is a non-contact measuring method which produces tensile stress acting on an interface by ultrasonic wave, the method enables us to evaluate the adhesion quality at high temperature. In this study, adhesion quality of the oxide scale on low carbon steel was attempted to evaluate at room temperature and up to 200 °C with the laser spallation technique. The exfoliation which was induced by the laser spallation technique can be identified by change in correlation coefficient for a series of the waveform obtained at each energy of incident YAG laser. Tensile stress acting on interface was estimated from stress distribution of the wave propagating through the interface calculated with a numerical simulation. As a result, it was confirmed that the adhesion quality of the oxide scale increased with testing temperature. It may be because the residual stress which acts on an oxide scale/substrate interface decreased with temperature.

Ultra-high Speed Imaging of Laser-Induced Spallation

In this work real-time images of the initiation and arrest of a laser-induced substrate spallation event have been captured over its ~40 ns duration using ultra-high speed photography. A modified version of the laser spallation technique is presented, introducing additional optical components allowing for control of the laser loading fluence (energy/unit area) without changing the emitted loading pulse energy, spot size, and profile. Spallation experiments were conducted on (100) silicon wafer substrates, 500 μm thick, coated with a 100 nm thick Al thin film at three fluence levels, 0.124, 0.174 and 0.336 J/mm2. The capabilities of the modified laser spallation technique capture spallation area evolution and spallation front velocity results over time are reported for the fluence levels examined. Following the spallation event, stress wave reflections and residual thin film stresses drive additional film delamination and substrate failure. Comparisons are made between the real-time spallation images and postmortem static images highlighting the effects of stress wave reflections.

OS0408 レーザスポレーション法を用いた炭素鋼における酸化スケールの高温密着性状評価

OS0408 レーザスポレーション法を用いた炭素鋼における酸化スケールの高温密着性状評価

Interfacial adhesion of photodefinable polyimide films on passivated silicon

Photodefinable polyimide films are used as dielectrics in flip chip integrated circuit packages to reduce the stress between silicon passivation layers and mold compound. Due to the disparity of material properties between the polymer and passivated substrates, interfacial delamination is a reliability concern. Understanding the effects of surface layers and film processing on polyimide adhesion is critical for development of more reliable packages. In this paper, we examine the influence of different cure cycles and UV-exposure on the adhesion between a photodefinable polyimide (HD4100 HD Microsystems) and silicon (Si) substrates with three different passivation layers: silicon nitride (SiNx), silicon oxynitride (SiOxNy), and the native silicon oxide (SiO2). The tensile strength of the thin film interface is measured by a laser spallation technique. Comparison of interfacial strength associated with cure cycle and UV-exposure indicates increased interfacial strength when films are processed with the exposure step as well as a longer cure cycle. Substrates including an oxygenated passivation layer have improved adhesion over nitride passivation layers. Additionally, the interfacial fracture energy is assessed using a dynamic delamination protocol. The interfacial fracture energy for this commercially available photodefinable polyimide on silicon is on the order of 100J/m2. The high toughness of this interface makes it difficult to use more conventional interfacial fracture testing techniques.