Introduction
Thin films are in use since ages, and in old times people used to beat gold with hammers into thin films about less than one μm thickness and then used this thin film for gold coating of different stuffs. Thin films are used in many materials and do use them in daily life but are not aware of it. If one wax cars or paint homes that mean, they are applying a thin film.
Bulk Material VS thin film
Bulk materials are those materials that are mined. They are referred to as Bulk solid in mining and mineral industry. The material that can be stored in bin or silo is also termed as bulk solids. So bulk material is a generalized term that elaborates a wide range of materials having varying sizes ranging from sub-micron powders to mine-run ores and having large boulders varying chemically as sugar, cocoa powder, limestone, coal, cement, sawdust, plastic pellets and metal shavings. Bulk materials are softer in terms of a lower modulus and hardness, showing a classical Hall-Petch relationship between the and grain size and yield stress. The lower elastic modulus of thick materials is due to the high prosperity and partially due to the surface roughness.
The handling of bulk materials or bulk solids includes mainly transportation, transfer, transshipment and storage. They have peculiar behavior that can be characterized in the context of flow function, the internal and wall friction angles, the flow factor of the material, and these characteristics can be easily measured by a shear tester. In order to find out the different characteristics of bulk materials, different test was conducted in the past. Some of the common properties of bulk materials include abrasiveness, adhesion, adhesiveness, aeration, air retention, angle of response and so forth.
Thin film is a layer of material that range from fractions of a nanometer to several micrometers in thickness. A common example is the household mirror that has a thin film coating on the back of a glass sheet to make a reflective interface. The elastic moduli of both bulk materials and thin films are not much different, but the parameters of plastic deformation can be larger than bulk materials. Thus, thin films can take much stress. A very critical property of thin films is that if one deposit a brittle material as Si on a flexible substrate, the substrate can be rolled up, and the thin film will not break. Bulk materials that appear opaque as the absorption length of light are shorter, so it becomes completely transparent like a thin film. The conductivity decreases in thin films as compared to thick materials. The yield stress of thin films is much higher than bulk material as thin films are nanocrystalline.
According to Kamiya, Kimuraa, Sakaa, and Abe pointed out that the law of similarity for the deformation of materials. It facilitates to presume the results for the samplings with different measurements in case of energy release rate, in a certain sample as presented figure 1. (Kamiya et al.) The overhanging film width could be considered with larger length even at infinite size. The similarity of deformation relative the density and size of films, the energy rate of samplings with approximate dimensions can be relied on the basis of the computation outcome for the model with the regular measurements. Moreover, the energy release rate is directly proportional to 2nd power of the load and the –ve 4th power of the thickness (Kamiya, Kimuraa, Sakaa, and Abe 180-186).
Figure 1 Relation b/w energy release rate and crack length
Source adopt: Kamiya, Kimuraa, Sakaa, and Abe 180-186
Toughness Measurement Methods
Toughness of any material is the capability to absorb energy in the process of deformation up to the fracture, and, the fracture toughness is the property of a material to withstand resistance developed by a preexisting crack (Zhang, Sun, Fu, and Du 74-84). So the toughness is a broad term that encompasses the energy required to create the crack as well as to enable it to propagate until fracture while the fracture toughness considers only the required energy to facilitate the crack propagation to fracture. Fracture toughness measurement for thin films is hard due to thickness limitations (Takahashi, Kamiya, Saka, and Abe 760-764).
Following are the methods used to measure the toughness of thin films:
- Bending
- Buckling
- Scratching
- Indentation
- Tensile Tests
For thin films having thickness of tens or hundreds of microns, the toughness measurement can be very much similar to thick materials. It works by creating a pre-crack, applying stress to induce crack propagation and to measure the critical stress needed to inflict fracture. However the introduction of pre-crack in thin films is very tricky part. Zhang, Sun, Fu, and Du and different other researchers proposed different mechanisms of bending. A diamond film having thickness in the order of millimeters was laser-cut at one edge in order to form a notch and then glued onto the side face of brass beam. This brass beam was bent to generate a pre-crack in the thin film at the ends of the notch. The film is then removed from the beam and to make it bend, it was put under a three-point flexure. Thus the fracture toughness was then used to be calculated by using the ASTM standard:
Kic = (PcS/hW2/3) f (a/W)
Pc = the load at fracture point; W is used for the width, and h referred the thickness of the thin film; S denoted the span between two support positions, and "a" is the length of the crack, while" is denoted for function of a/W. Due to difficulty in gluing and ungluing, propagation of a crack in brass plates and so forth, this method is not applied for thin films.
Zhang, Sun, Fu, and Du have introduced different ways of fracture toughness measurement using bending. The whole phenomena circulate around the creation of pre-crack. They constructed a notched side of the steel substrate; and then a hole was bored in the end. The substrate was fatigued in order to develop a crack from the notch that consequently propagated and stopped at the hole. A thick film is then deposited at the side faces of the substrate of about few micron thicknesses. This coated substrate then goes for two successive four-point bending test. The load was then recorded as a function of displacement while bending (Zhang, Sun, Fu, and Du 74-84).
Takahashi, Kamiya, Saka, and Abe, and some other researchers proposed another method of fracture toughness measurement called as buckling especially for indium-tin-oxide (ITO). They used Polyethylene Telephthalate (PET) as a substrate that was used due to its elastic nature. ITO thin films with thickness between 80 – 140 nm were deposited on PET substrate. According to buckling deformation theorem of beam, the equation obtained was:
X = 2[1 – E (k) / K (k)] L/R = 4K (k) k
The K (k) and E (k) are “elliptic integrals”, k = sin (θ / 2), L is the length of the beam, R is denoting the radius of curvature, and x = e / L is the contraction ratio. (Takahashi, Kamiya, Saka, and Abe 760-764; Zhang, Sun, Fu, and Du 74-84).
In the case of scratching form of testing, a diamond made tip is driven over a coated surface to generate a scratch. The load on the diamond tip is slowly increased in order to induce a shear force in the nearby film which is proportional to the load applied and then transmitted through the bulk of the composite sample.The discontinuity in the shear stress at the interface can be observed due to the difference in the mechanical properties of the substrate and film. For hard thin films, minute micro-cracks appear in the film while scratching process. So the minimum load at which the first crack appears is called as the lower critical load Lc1 and the load that results in the complete peeling of the film is called as the higher critical load Lc2. The lower critical load is termed by some researchers as the scratch toughness (Choi, Lee, Wang, and Oh 38-43).
The indentation is most commonly used method for the measurement of the toughness of thin films. Plastic deformation of materials leads to stress relaxation in them. If the stress relaxation proceeds easily, the larger plasticity will be inherent in the material. Comparison of plastic strain with the total strain in an indentation test give tough, but quick way of measuring regarding toughness of the material. In order to measure the toughness of thin films using this method, a pre-crack into the film is introduced using focused ion beam milling. The crack opening force is generated with the help of indentation sink-in effect. This sink-in effect gives a tensile stress on the film near the pre-crack tip, and this promotes the crack propagation. A Knoop indenter is used here to induce a plane strain condition near the indenter. The sink-in effect is larger in the center of the indentation and decreases along the edges of the indentation. Under plain stress, fracture toughness can be expressed as a function of crack tip immediately before the catastrophic failure with the help of the equation as under:
KIc = (mδσyE) ½
"m" is a dimensionless constant having value 2.90 for a plane stress condition, "σy" represent yield stress, and E is the "Young's Modulus of the Thin film", δ is the crack tip opening distance and this is the amount of crack tip blunting before the catastrophic growth (Choi, Lee, Wang, and Oh 38-43).
For relatively thick films, the fracture toughness can be measured directly by tensile testing. In this method, a precrack is introduced by laser cutting. The fracture toughness of the thin film is measured as 5 – 6 MPa m1/2 as compared to the indentation method. However, Nose, Sasaki, Kamiko, and Mitsuda proposed that this measurement be more sensible without creating the pre-crack, because, of making precracks in micron or submicron thin films is difficult and generate uncertainties. They proposed a micro-tensile method in which a flat rectangular substrate being coated with a film is put under tension. This cracking of the film causes energy variation in the film/substrate system. The toughness of the film is then calculated based on energy balance as soon the cracking occurs (Nose, Sasaki, Kamiko, and Mitsuda 1-5).
The mechanical properties and microstructure are important parameters in order to measure the fracture behavior and reliability of thin films. The mechanical properties of thin film tend to present different behavior comparative to bulking materials. That can be partially elaborated by the concept of the nanocrystalline structure can be attributed with substrate for thin films. The high values of residual stresses leads to high yield strength and these can be relieved during the process or by interfacial delaminating
Plastic and elastic properties of thin film have a significant role in behavior of fracture. The mechanical properties can be evaluated by applying micro beam deflection or tensile testing of detached films. On another hand, nanoindentation is likely to traditional hardness testing that applied by using very sensitive loads at very smaller scale ( Volinsky,Vella,Adhihetty, and et al ).
Fracture Behavior of Thin Film (DLC) Deposited on Different Substrates
In this review, effects of aluminum alloy, steel, silicon and ceramic are studied to evaluate the effectiveness of substrate on thin film fracture behavior in different conditions.
Effect of Aluminum Alloy DLC Film
Since DLC films achieve good anti-adherent properties without even using of any lubricant, DLC film is considered as the perfect coating for aluminum alloy for dry machining. Researchers are trying to find out the applications of DLC in cutting tools.
Nose, Sasaki, Kamiko, and Mitsuda studied the fracture behaviors of a diamond like carbon (DLC) film with an aluminum alloy .They analyzed it by a nano indentation test by considering conditions of full and deep penetrations of an indenter tip via DLC film. The line structure between aluminum alloy and the diamond-like carbon was modified by using the substrate to evaluate the effects on fracture behavior. The films deposited with a period of 30 min showed weak adhesion to the substrate and resulted in wide delaminating from the impressions. In addition, the film deposited during the period of 120 min showed no such delaminating. They found few brittle fractures in the form of load displacement curves in the film with a short duration. According to these results, they proved that a long SSRD could inhibit the delaminating of the DLC film from aluminum alloy under local and strong stress conditions due to improved interface toughness by application aluminum substrate on the thin films (Fukui, Okida, Omori, and et al. 70-76; Nose, Sasaki, Kamiko, and Mitsuda 1-5).
The phenomenon of the delimitation of samples is associated with a comparatively low energy release rate that implied weak interface toughness between substrate and DLC films. The different pattern of fracture exhibits different behavior after the indentation. They pointed that brightness can be obtained in electrically charged regions of film delamination from the substrate by virtue of dielectric property of DLC. It was predicted that the fracture developed in multiple phases. They further observed that delaminated region is limited in the context of width in peripheries of the impression developed in the substrate samples and interestingly, no buckling was observed in the samples.
Figure 2 Load displacement curves for SSRD
DLC Film on Stainless Steel
DLC film coating on stainless steel produces less frictional resistance than as found for as-received wires. Several studies have been conducted to investigate the effect of DLC coating on the frictional properties of stainless steel brackets and it is hypothesized that a DLC coating never affects the frictional properties stainless steel brackets ((Muguruma, Takeshi, Iijima Masahiro, Brantley A. William, and e.t al.2-7).
When the DLC film is coated on stainless steel, the tensile deformation is progressed and the crack of the film is observed in the perpendicular direction to the tensile axis. Further deformation then increases both the cracks and the spallation. Porosity and corrosion density increases and the protective efficiency decreases at the strain of 2%. Instead of the degradation, the anti corrosion properties are significantly improved as compared to the uncoated stainless steel. The significant increase in the porosity and corrosion density is observed at the strain of 4% (Choi et al, 2005).
Figure 3 Schematic presentation of tensile for steel
Source adopted: Choi, Lee, Wang, and Oh 38-43
DLC Film on Ceramic
A DLC film has the capability to improve tribological properties of silicon nitride ceramic elements that consequently affect the fracture behavior of thin films. Research has been conducted for preparation and analysis of DLC films on rings and discs. However a very little research has been conducted on fabrication of DLC films and evaluation of effectiveness of this substrate. It is hard to achieve uniform coating on ball surfaces. DLC films are deposited on ceramic ball surfaces with plasma immersion ion implantation and deposition technique which generated uniform coatings on all ball surfaces (Zhang, GU and Zheng 63-65).
For the DLC coatings on ceramics, it is possible to predict qualitatively the adhesion strength in terms of the critical load. DLC films started cracking but never delaminated from the substrate. This shows that the DLC film adheres well to the ceramic polycarbonate. The film cracks as the indenter tip moves into the surface of the ceramic substrate. Thus the stress built up ahead of the tip is relieved by the cracking of the film and never becomes large to delaminate the film (Choi et al, 2005).
Rasel, Wang, Ku and et al. studied the surface morphology to analyze the fracture behavior of DLC coated surface. They observed that well spread carbon grains throughout the surface making sp3 bond that gives high hardness for coating surface. Moreover, they analyzed the area SEM by EDS images for ZrO2 and the presences of amorphous carbon on this substrate. It could be identified the compositions of substrate materials. Vicker's hardness for ZrO2 and DLC coated ZrO2 of various thicknesses under normally applied load of two kgf is shown in Figure 4. That Figure presented observed hardness values that improved significantly by increasing the thickness. Comparative analyses based on materials showed 3.37%, 5.54%, and 8.48% hardness improvements against values of 6.4 — 40.4, and 53.2 μm DLC coated ZrO2 materials — respectively. DLC film coating on stainless steel produces less frictional resistance and improves the effectiveness of substrate. The Vickers indentation tests carried out on ceramic (ZrO2) revealed that the fracture behavior for toughness present enhanced values on DLC coating. In addition, the maximum value for fracture toughness can be obtained by applying 53.2 µm coating on DLC materials (Rasel, Wang, Ku and et al.).
Figure 4 Vicker hardness of ZrO2 and DL Coated with ZrO2 having different thickness
Source adopted: Rasel, Wang, Ku and et al
DLC Film on Silicon
The toughness of interface had varying effects in case of diamond films on silicon substrate but it can be controlled for effective output by using methane concentration. The mechanical properties can be improved by adhering toughness of diamond particles on silicon substrates. It was observed that the adhesive toughness of diamond film was not much significant comparative to diamond particle that demands further research and investigations to evaluate behavior of fracture for thin films .Moreover, DLC coating that was done on crystalline silicon chips (solar cell material) greatly reduced the reflection of incoming radiation and enhancing the efficiency of solar cells (Takahashi , Kamiya, Saka, and Abe 760-764).
Conclusion
Diamond like carbons (DLC) films are known by their low friction coefficients, high wear resistance and high corrosion resistance. As per testing environment, the coefficient of friction can be as low as 0.01. DLC coatings are the field of intensive research these days. Unfortunately; there, is yet no standard procedure to measure the toughness of thin materials, however, research is still on to address this complicated issue and hence few of the test have been proposed by different scientists. DLC has many applications in engineering and medical field. It has been used as a topping on different materials like aluminum alloy, silicon, ceramic, stainless steel etc and thus showing different fracture behavior. Although the standard method for toughness measurement of thin films have not been finalized yet, but still different techniques are being used to measure the toughness of thin films including buckling test, indentation test, tensile test and scratching test in order to evaluate fracture behavior on thin films.
Work Cited
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