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Keywords:

  • Plasma Spray;
  • Iron Boride Coating;
  • Indentation;
  • Mechanical Properties

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References

In this work, we draw attention to determination of heat-treatment effects on mechanical properties of atmospheric plasma sprayed (APS) FexB coatings on aluminum substrates by micro-indentation technique. With this regard, iron boride powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were deposited onto Al substrates by APS in order to improve the mechanical properties of Al surface. As-sprayed coatings are composed of mainly FexB and iron matrix supersaturated with boron owing to the rapid solidification of molten droplets flattened on a substrate. It was observed that APS coatings exhibited characteristic wavy layered structure having porosity, inclusions, and semi-melted particles. The postfurnace treatment of APS coatings was carried out at temperatures ranged from 450 to 550°C in an argon atmosphere. The post-treatment applied for APS deposits led to increase in hardness of 40% without showing cracks. Furthermore, micro-mechanical properties of FexB coatings were examined by Shimadzu Dynamic Ultra-Microhardness Tester for estimating Young's modulus and hardness due to load–unload sensing analysis by applying different loads such as 160, 320, and 640 mN to determine load and indentation depth dependency of APS FexB on Al substrate for each samples, in details.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References

Boriding is a thermomechanical surface-hardening process, in which boron atoms are diffused into the surface of a workpiece to form borides with the base materials. Industrial boriding can be applied to most ferrous materials, such as structural steels, cast steels, Armco iron, gray-ductile iron, sintered iron, steel, as well as to nonferrous materials, such as nickel-, cobalt-, titanium-, and molybdenum-based alloys and cemented carbides.1 The major advantage of a modified surface layer by deposition of boride layers is its high-wear resistance.2 It is well known that conventional boriding process, that is, pack boriding, is commonly applied in the temperature range 1073–1273 K. Depending on the process temperature, chemical composition of substrate materials, several hours are required in order to obtain desired hardened surface layer by formation of interstitial boron compounds. The resulting layer may consist of either a single-phase boride (FeB or Fe2B) or polyphase boride layer (FeB and Fe2B). However, it is almost impossible to produce hardened surface composed of only single phase preferably Fe2B during boriding. The formation of FeB layer is inevitable but undesirable phase because it is very brittle and has higher thermal coefficient (15 × 10−6 K−1) than that of Fe2B (8 × 10−6 K−1) which leads to develop thermal stresses during heat cycle. Recently, it has been shown that the aforementioned disadvantages of conventional boriding process could be eliminated by introducing fast deposition processes such as atmospheric, vacuum plasma spraying, and plasma transferred arc (PTA) boriding.3 Plasma spraying is an attractive coating method because it offers fast and cost-effective solutions for the problems of wear, corrosion, and thermal stability by depositing a thin layer onto the substrate, thereby satisfying the required surface specification.

For instance, it is difficult to develop a diffusion iron boride layer on the aluminum surface by conventional boriding process. Plasma spraying of iron boride powder successfully performed with a thickness of hundreds microns onto aluminum surfaces which exhibited good antiwear performance.4 Likewise, Galvenetto et al.2 modified carbon steel surfaces by depositing iron boride layers with graded composition by VPS technique to improve their wear resistance.

During the past quarter of a century instrumented indentation machines have been used increasingly for determining Young's modulus and indentation hardness of bulk solids and thick coatings deposited on solid substrates. A typical experimental run with an instrumented indentation machine consists of loading normally a Vickers or Berkovich indenter on to the test surface and gradually increasing the load at a predetermined rate to a preselected value and then unloading the indenter gradually to zero load. Throughout the indenter loading and unloading the indenter load versus indenter displacement with respect to the original surface of the specimen are recorded. In the case of an elastic–plastic solid, plastic flow will occur around the pointed indenter and when the indenter is unloaded and removed from the indented surface, a permanent impression will be left in the surface of the specimen (Fig. 1).3

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Figure 1. Schematic illustration of a typical Ph response of an elasto-plastic material to instrumented sharp indentation.

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Five means are commonly used for coating Young's modulus measurement: (1) nanoindentation, (2) beam bending, (3) vibration, (4) ultrasonic surface wave emission, and (5) acoustic emission.5–7 Among these methods, indentation is the most popular technique, with which Young's modulus is often determined through the initial part of the unloading curve of a set of indentation data obtained with a sharp indenter.8–12 The depth-sensing (or dynamic) micro-indentation method offers great advantages over the conventional Vickers microhardness testing in two aspects. Firstly, apart from microhardness (or microstrength), the method can also provide well-defined mechanical parameters such as elastic modulus of the interfacial zone. Secondly, as load and depth of an indentation are continuously monitored, optical observation and measurement of diagonal length of the indent/impression, which can be difficult and subjected to inaccuracy, is no longer required.13,14

In this study, we draw attention to microstructural and thermomechanical properties of FexB coatings on Al substrates. In this context, FexB-based coatings were fabricated on Al substrates by using atmospheric plasma spray (APS) technique. The produced coatings were characterized by scanning electron microscope (SEM). Mechanical properties (Hardness and Young's Modulus) of FexB coatings were obtained from micro-indentation tests. The as-sprayed layers were subjected to post-spray heat-treatment process in a temperature ranging from 450 to 550°C for 1 to 2 h in an argon atmosphere in order to improve mechanical properties of sprayed coatings. The dynamical hardness measurements were examined to determine modulus and hardness variations of boride layer under different applied peak loads. With this regard, it was aimed to show the load and indentation size dependency of the elasto-plastic properties of plasma sprayed FexB coating on Al substrate by micro-indentation technique.

Experimental Study

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References

The FexB powders with a composition of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) were sprayed onto grit blasted 30 × 30 × 5 mm aluminum alloy substrate (die cast, Al-11.2Si-2.74Cu; in wt.%) by APS method. The standard spray parameters followed for deposition is given in elsewhere.15 In order to quantitatively determine the level of porosity, an image analyzer (LUCIA 4.21) was used. The Vickers hardness of the coating layers was measured with a load of 2.94 N.

The surface morphologies of layers were examined by a Scanning Electron Microscope (JEOL-JSM 6060 SEM (JEOL, Tokyo, Japan)). Accelerating voltage of 20 kV was used for the SEM imaging analyses. Mechanical properties (hardness and young's modulus) of FexB were obtained from load-depth curves by using Shimadzu Dynamic Ultra-Micro Hardness tester (Shimadzu DUH-W210S, SHIMADZU, Kyoto, Japan) under 160, 320, and 640 mN applied peak loads. In order to determine the Young's modulus and hardness, maximum applied load should be sufficient to produce a permanent deformation on the coating surface.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References

Microstructure of the coatings

Coatings sprayed by APS exhibited characteristic wavy, layered structure without showing cracks inside the layers. The average microhardness value of the iron boride layers is about 760 HV which is much lower than borided layers of AISI 1020 steel at 900°C for 4 h. Typical cross-section microstructure of the coatings sprayed with these parameters is shown in Fig. 2, where the sprayed coating exhibits a wavy layered structure and consists of some unmelted particles and pores, with an average thickness of approximately 100 µm. X-ray diffraction patterns of as-sprayed and heat-treated samples are shown in Fig. 3. As seen at higher magnification (Fig. 4(b)), some microcracks are observable inside the layers. In order to eliminate these microcracks and, preheating substrate before spraying, stopping compressed air cooling used for substrate cooling from the backside as well as extending spray distance from 150 to 170 mm were also tried. However, formation of microcracks was generally not avoided. This is attributed to the cooling rate which leads to generation of residual stresses, which might result in cracks in the coating. PTA boriding study conducted by Bourithis et al.16 showed that microcracks were observed inside the borided layers with increasing content of boron, which lead to the local formation of FeB and, eventually, intergranular cracking. Galvenetto et al.2 sprayed Fe2B (%100) and FeB+α-Fe graded powder composition by VPS on AISI 1040 steel, it was concluded that no cracks were observed inside the iron boride layers. Both studies were focused on fast formation of boride layers on low carbon steel which used commonly for pack boriding process.

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Figure 2. SEM image of the as-sprayed coatings by APS.

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Figure 3. XRD results of as-sprayed and heat-treated samples.

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Figure 4. SEM images of cross-section microstructures of the coatings sprayed by APS (a) as-sprayed and post-treated at (b) 450°C for 1 h, (c) 500°C for 2 h, and (d) 550°C for 2 h.

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While, in this study, aluminum surface was modified by depositing boride layers by APS, which its surface is not possible boriding in conventional boronizing. The observed cracks inside the coating produced by APS might be attributed to fast cooling during spray process due to the higher thermal conductivity of Al compared to low carbon steel, such cracks might be eliminated by spraying graded composition of iron boride powders. Postheat treatment process was applied to treat the sprayed boride layers, enabling the boron diffusion which decomposed and formed solid solution during spray process. In order to densify and improve the mechanical properties, the coatings were post-treated in a furnace at temperatures of 450–550°C for 60 to 120 min in an atmosphere of argon. Modification of initial coating microstructure is shown in Fig. 4. Post-treated microstructure became smoother and improved exhibiting decrease in pore size, that is, coating densification, better adhesion of splat layers as well. As a matter of fact that the hardness of predeposited coating increased significantly at the rate of 40%. Depending on composition of borided layers, measured hardness value changes from 400 to 1600 HV. The measured hardness values of as-sprayed and post-treated coatings are much lower compared with data of borided AISI 1020 steel at 900°C for 4 h, yielding maximum hardness value, 1600 HV. On the other hand, several studies showed that iron-based powder sprayed coatings exhibited good antiwear performance.17,18 The improvement at wear performance and reduction in friction coefficient not only depend on coating hardness but also on the formation of protective oxide films which strongly affect the wear mechanisms of the layers. Therefore, coating sprayed with ferroboron powder containing FexB intermetallic phase, boride oxide, and Fe-based oxide expected to experience with low friction coefficient and improved wear resistance in comparison to the steel surface which is conventionally boronized. Moreover, contrary to the classical boronizing, thermal spraying is a cost-effective promising technique to solve problems such as wear, corrosion, and thermal stability by producing rapidly solidified thick materials on any substrate with a relatively short cycle time.

Mechanical properties of coatings

Dynamic Ultra-microhardness test is applied to cross-section part of samples for determination of hardness and Young's modulus variations of FexB under the loads of 640, 320, and 160 mN applied loads. Dynamic Hardness results for without heat treatment, 450°C for 1 h, 500°C for 1 h, and 550°C for 2 h under 640, 320, and 160 mN applied peak loads are shown in Fig. 5. The figures were constructed experimentally using the data taken from the loading part of depth setting Dynamic Hardness Values (DHV) measurements at various loads. It is seen that DHV numbers decrease with increasing applied peak load and indentation depth. Hardness measurements of FexB shows that when the heat treatment process temperature increase from non-heat treated to 550°C, DHV of FexB layer decreases from 1518 DHV to 657 DHV, from 1042 DHV to 475 DHV, from 897 DHV to 653 DHV, and finally from 953 DHV to 245 DHV with increasing applied peak loads from 160 to 640 mN. As hardness is accepted as an inherent material property, it should not vary with indentation load and size. However, investigations have confirmed that DHV number of materials were indentation size dependent especially at lower peak loads. Increase in hardness with decreasing applied peak load cause from differences in indentation depth, therefore this effect is called indentation size effect. When the curves are examined, two different parts of hardness variation can be clearly seen. The first part of the curve represents the contact region between the indenter and coatings. So, at this point, indentation depth influences the dynamic hardness value of coatings at near the surface region. When the applied load and depth increase, depth effect systematically decreases. At the second part of the curve, dynamic hardness value of coatings is approximately constant depending on indentation depth. The first stage of curves shows the load and size dependency of hardness under applied peak loads. Figure 5 exhibits this kind of behavior.

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Figure 5. Hardness-penetration depth curves of the coatings sprayed by APS as-sprayed and post-treated at 450°C for 1 h, 500°C for 1 h, 550°C for 2 h under (a) 160 mN, (b) 320 mN, and (c) 640 mN applied peak loads.

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In order to determine the Young's modulus, maximum applied load should be sufficient to produce a permanent deformation on the coating surface. So that the values of Young's modulus, determined from indentations, do not depend on the value of h and, therefore, on the value of the maximum load, the indentation depth should not exceed 10–20% of the layer thickness, otherwise the results will be affected by the properties of the substrate. In this study, thicknesses of FexB layers are between 442 and 298 µm depending on heat-treatment conditions and maximum indentation depth changes among 3.19 and 0.627 µm.

The load–unload (load–displacement) curves shown in Fig. 6 represent the 640, 320, and 160 mN applied load as a function of the displacement of the indenter with respect to the initial position of the cross-section part of layers. Using the experimentally determined Sand hc, the reduced elastic modulus by micro-indentation was calculated and the results are shown in Fig. 7 for heat-treated and non-heat-treated samples. It is clearly seen from the figures that the extracted reduced elastic modulus also exhibits a strong peak load dependency as shown in Table 1. According to the result, Young's modulus values increase with decreasing applied peak loads. The Young's modulus analysis of FexB layer shows that when the heat-treatment temperature increases from non-heat treated to 550°C, Young's modulus of FexB layer decreases from 678 to 202 GPa, from 300 to 110 GPa, and finally from 235 to 52 GPa with increasing applied peak loads from 160 to 640 mN.

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Figure 6. A series of load-displacement plots of the coatings sprayed by APS (a) as-sprayed and post-treated at (b) 450°C for 1 h, (c) 500°C for 2 h, and (d) 550°C for 2 h under 160, 320, and 640 mN applied peak loads.

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Figure 7. Young's modulus variations of as-sprayed and post-treated at 450°C for 1 h, 500°C for 2 h, and 550°C for 2 h under 160, 320, and 640 mN applied peak loads.

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Table 1. Dynamic hardness results of samples
Material typeLoad (mN)Maximum depth (µm) h c (µm)Dynamic hardness value (DHV)Young's modulus (GPa)
Without heat treatment1600.6400.4321518678.34
 3201.1150.7761011300.07
 6401.9481.430657236.80
450°C–1 h1600.7720.5931042411.64
 3201.4171.058627181.26
 6402.2891.791475154.84
500°C–1 h1600.8390.708897352.40
 3201.2160.767836174.65
 6401.9441.169653132.30
550°C–2 h1600.8140.457953202.30
 3201.4450.844600110.90
 6403.1902.21124552.27

Furthermore, elasto-plastic properties of coatings, such as C constant, yield strength, indentation hardness, and strain hardening exponent variation of FexB coatings depending on applied loads and with–without heat treatment was calculated by indentation algorithms, as represented Fig. 8. According to the results, C constants, yield strengths, indentation hardness's, and strain hardening exponents of FeB coatings were decreased from 325 to 76 GPa, from 4.62 to 1.15 GPa, from 27.95 to 4.41, and finally from 0.2621 to 0.2600 by increasing applied load from 160 to 640 mN and heat-treatment temperature from 450 to 550°C, respectively. Indentation size and heat-treatment effect on elasto-plastic properties of FexB coatings were clearly seen in Fig. 7. According to the several studies,19,20 load depended elastic modulus (125–624 GPa) and hardness (17–33 GPa) were obtained at 80, 160, 320, and 640 mN applied peak loads depending on boriding process time. In addition, finite element modeling was applied to simulate the mechanical response of FexB layer on low alloy steel substrate. When the results of mechanical properties were compared from current studies, it can be considered that indentation hardness, Young's modulus, and elasto-plastic properties of coatings show similarities. When the diffusion-controlled production regime and atmospheric plasma spraying compared each other, FexB layer on different substrate shows same indentation properties.20,21

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Figure 8. Indentation hardness and strain hardening exponent variations of coatings.

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As known, indentation size effect played an important role while increasing indentation loads and depths. In addition to this effect, heat treatment at 450–550°C at inert atmosphere influenced the indentation algorithm results. Concurrently, comprehensive theoretical and computational studies have emerged to elucidate the deformation mechanism in order to systematically extract material properties from P versus h curves obtained from instrumented indentation. Analysis algorithms for determination of elasto-plastic properties were established based on the identified dimensionless functions.21 These algorithms allow for the calculation of the indentation response for a given set of properties, and also for extraction of yield strengths and strain hardening exponent from a given set of indentation data. The main aim of heat treatment was to obtain the improvement of mechanical properties of FexB coatings on Al substrate by APS. Young's modulus, yield strength, indentation hardness, and strain hardening exponents of coatings systematically decreased and mechanically diverged from brittle behavior. In addition, residual stresses and mechanical mismatch effect of Al substrate-FeB coating was also decreased due to the heat treatments.

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References

In this paper, the authors were deposited iron boride powders of Fe-18.8B-0.2C-0.5Si-0.8Al (wt.%) onto an Al substrate by APS. The microstructure and mechanical impact phenomenon of the coating was reached as below:

  • Iron boride layers deposited by APS showed characteristic wavy, layered structure without showing cracks. Iron boride coating is mainly is consisted of FexB phase according to the stoichiometric powder mixing.

  • Smoother and improved microstructure, that is, decrease in pore size, coating densification were obtained after post-treatment of APS coating. The measured hardness of post-treated coating increased remarkably at the rate of 40%.

  • Hardness measurements of FexB coatings shows that when the heat-treatment process temperature increase from non-heat treated to 550°C, DHV of FexB decreases from 1518 DHV to 657 DHV, from 1042 DHV to 475 DHV, from 897 DHV to 653 DHV, and finally from 953 DHV to 245 DHV with increasing applied peak loads from 160 to 640 mN.

  • The Young's modulus analysis of FexB layer shows that when the heat-treatment temperature increases from non-heat treated to 550°C, Young's modulus of FexB layer decreases from 678 to 202 GPa, from 300 to 110 GPa, and finally from 235 to 52 GPa with increasing applied peak loads from 160 to 640 mN.

  • Indentation properties of FexB coatings such as C constants, yield strengths, indentation hardness's, and strain hardening exponents of FexB coatings were decreased from 325 to 76 GPa, from 4.62 to 1.15 GPa, from 27.95 to 4.41, and finally from 0.2621 to 0.2600 by increasing applied load from 160 to 640 mN and heat-treatment temperature from 450 to 550°C, respectively.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental Study
  5. Results and Discussion
  6. Conclusion
  7. References
  • 1
    Ozbek, I., Bindal, C., “Mechanical Properties of Bronzed AISI W4 Steel,” Surface and Coatings Technology 154:1420 (2002).
  • 2
    Galvanetto, E., Borgioli, F., Bacci, T., Pradelli, G., “Wear Behaviour of Iron Boride Coatings Produced by VPS Technique on Carbon Steels,” Wear 260: 825831 (2006).
  • 3
    Lim, Y.Y., Chaudhri, M.M., “Indentation of Elastic Solids With a Rigid Vickers Pyramidal Indenter,” Mechanics of Materials 38:12131228 (2006).
  • 4
    Ozdemir, I., “Wear/Friction Behaviour of FeB and FeB/h-BN Coatings Sprayed by Atmospheric Plasma Spraying,” Praktische Metallographie 44:355371 (2007).
  • 5
    Schneider, D., Schwarz, T., Schultrich, B., “Determination of Elastic-Modulus and Thickness of Surface-Layers by Ultrasonic Surface-Waves,” Thin Solid Films 219:92102 (1999).
  • 6
    Alfano, M., Pagnotta, L., “Measurement of the Dynamic Elastic Properties of a Thin Coating,” Review of Scientific Instruments 77:056107056107/3 (2006).
  • 7
    Liu, S.B., Wang, Q.J., “Determination of Young's Modulus and Poisson's Ratio for Coatings,” Surface and Coatings Technology 201:64706477 (2007).
  • 8
    Loubet, J.L., Georges, J.M., Marchesini, O., Meille, G., “Vickers Indentation Curves of Magnesium-Oxide,” Journal of Tribology 106:4348 (1984).
  • 9
    Doerner, M.F., Nix, W.D., “A Method for Interpreting the Data from Depth-Sensing Indentation Instruments,” Journal of Materials Research 1:601609 (1986).
  • 10
    Pharr, G.M., Oliver, W.C., Brotzen, F.R., “On the Generally of the Relationship Among Contact Stiffness, Contact Area, and Elastic-Modulus During Indentation,” Journal of Materials Research 7:613617 (1992).
  • 11
    Oliver, W.C., Pharr, G.M., “An Improved Technique for Determining Hardness and Elastic-Modulus Using Load and Displacement Sensing Indentation Experiments,” Journal of Materials Research 7: 15641583 (1992).
  • 12
    Maxwell, A.S., Owen-Jones, S., Jennett, N.M., “Measurement of Young's Modulus and Poisson's Ratio of Thin Coatings Using Impact Excitation and Depth-Sensing Indentation,” Review of Scientific Instruments 75:970975 (2004).
  • 13
    Uzun, O., Kolemen, U., Celebi, S., Guclu, N., “Modulus and Hardness Evaluation of Polycrystalline Superconductors by Dynamic Microindentation Technique,” Journal of the European Ceramic Society 25:969977 (2005).
  • 14
    Culha, O., Celik, E., Ak Azem, N.F., Kayatekin, I., Toparli, M., Turk, A., “Microstructural, Thermal and Mechanical Properties of HVOF Sprayed Ni-Al-based Bond Coatings on Stainless Steel Substrate,” Journal of Materials Processing Technology 204:221230 (2008).
  • 15
    Ozdemir, I., Ueno, T., Tsunekawa, Y., Okumiya, M., “Cast iron coatings containing graphite structure by atmospheric plasma spraying,” Proceedings 2004 International Thermal Spray Conference, ITSC-2004, Osaka, Japan; May 10–12, 2004.
  • 16
    Bourithis, L., Papaefthymiou, S., Papadimitriou, G.D., “Plasma Transferred Arc Boriding of a Low Carbon Steel: Microstructure and Wear Properties,” Applied Surface Science 200:203218 (2002).
  • 17
    Hwang, B., Ahn, J., Lee, S., “Correlation of Microstructure and Wear Resistance of Ferrous Coatings Fabricated by Atmospheric Plasma Spraying,” Metallurgical and Materials Transactions A 33:29332945 (2002).
  • 18
    Edrisy, A., Perry, T., Cheng, Y.T., Alpas, A.T., “Wear of Thermal Spray Deposited Low Carbon Steel Coatings on Aluminum Alloys,” Wear 251: 10231033 (2001).
  • 19
    Culha, O., Toparlı, M., Sahin, S., Aksoy, T., “Characterization and Determination of FexB Layers' Mechanical Properties,” Journal of Materials Processing Technology 206:231240 (2008).
  • 20
    Culha, O., Toparlı, M., Aksoy, T., “Estimation of FeB Layer's Yield Strength by Comparison of Finite Element Modeling with Experimental Data,” Advances in Engineering Software 40:11401147 (2009).
  • 21
    Dao, M., Chollacoop, N., Van Vliet, K.J., Venkatesh, T.A., Suresh, S., “Computational Modeling of the Forward and Reverse Problems in Instrumented Sharp Indentation,” Acta Materialia 49:38993918 (2001).