pith. sign in

arxiv: 2607.01997 · v1 · pith:UO76LJHWnew · submitted 2026-07-02 · ❄️ cond-mat.mtrl-sci

HVAF Spraying of NiTi Coatings: Microstructure, Phase Transformation and Shape Memory Behavior

Sneha Samal , Shrikant Joshi , Stefan Bj\"orklund , Mohit Chandra , Akhil Bhardwaj , Jarom\'ir Kope\v{c}ek , Stanislav Habr , Luk\'a\v{s} V\'aclavek Jan Tom\'a\v{s}t\'ik
show 2 more authors
Ond\v{r}ej Tyc Petr \v{S}ittner
This is my paper

Pith reviewed 2026-07-03 09:45 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci
keywords NiTi coatingsHVAF sprayingshape memory alloysmartensitic transformationthermal spray coatingsmicrostructurephase transformationadhesion
0
0 comments X

The pith

High velocity air fuel spraying produces thick NiTi coatings that retain shape memory behavior and adhere well to steel.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper demonstrates that high velocity air fuel thermal spraying can deposit 100-300 micrometer thick NiTi layers on mild steel using four different parameter sets. These coatings undergo martensitic transformation upon cooling or loading after annealing, despite containing voids, oxides, dislocations and stresses, and they exhibit thermal actuation in bending plus shape memory in nanoindentation and scratch tests. Prior techniques either eliminated the transformation or produced weak adhesion, so this result matters for anyone seeking functional coatings that add actuation to engineering surfaces. A sympathetic reader would see a route to metallic coatings that combine wear resistance with reversible thermomechanical response.

Core claim

HVAF spraying with varied parameters yields thick NiTi coatings on mild steel that display chemical inhomogeneity, porosity, oxide particles, high dislocation density and internal stresses yet still perform martensitic transformation after annealing. The coatings show thermal actuation in three-point bending, shape memory recovery in nanoindentation and scratch tests, and very good adherence to the substrate.

What carries the argument

The HVAF thermal spray deposition process, which builds thick NiTi layers while leaving the alloy capable of reversible martensitic transformation after annealing.

If this is right

  • Annealing is required to activate the thermomechanical response in the as-sprayed coatings.
  • The coatings produce measurable thermal actuation under bending loads.
  • Shape memory effects appear in localized tests such as nanoindentation and scratching.
  • Adhesion to the mild steel substrate remains strong despite the internal stresses and defects.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The approach could be tested on substrates other than flat mild steel to check whether complex geometries retain the same transformation behavior.
  • Adjusting the four spray parameter sets might further reduce porosity while preserving the observed transformation temperatures.
  • Combining these coatings with conventional hard layers could be explored for parts needing both wear protection and actuation.

Load-bearing premise

Defects such as porosity, oxides and dislocations introduced by spraying still allow the NiTi to undergo reversible martensitic transformation upon cooling or loading.

What would settle it

Absence of reversible strain recovery in the three-point bending actuation tests or lack of martensitic transformation peaks in thermal analysis of the annealed coatings would show the functional properties are not achieved.

Figures

Figures reproduced from arXiv: 2607.01997 by Akhil Bhardwaj, Jarom\'ir Kope\v{c}ek, Luk\'a\v{s} V\'aclavek Jan Tom\'a\v{s}t\'ik, Mohit Chandra, Ond\v{r}ej Tyc, Petr \v{S}ittner, Shrikant Joshi, Sneha Samal, Stanislav Habr, Stefan Bj\"orklund.

Figure 1
Figure 1. Figure 1: NiTi powder characteristics a) Size and shape of the NiTi particles characterized by SEM, b). Histogram of powder size distribution (davg: 26.76 μm, d90: 37 µm), c) DSC scan of the NiTi powder performed to evaluate transformation temperatures (Ms =39 ºC, Af =62 ºC) d) red mark in the powder shows the location for chemical analysis, e) Detail of particle used to analyze chemical composition by EDX, f) Resul… view at source ↗
Figure 13
Figure 13. Figure 13: Local mechanical response of force versus displacement evaluated in nanoindentation tests on as￾sprayed HVAF coatings 1-4 performed using maximum loads 50. 100, 150, 200 mN [PITH_FULL_IMAGE:figures/full_fig_p019_13.png] view at source ↗
read the original abstract

Depositing various coatings on surface of engineering components with the aim to improve their performance concerning wear, corrosion, friction and thermal protection is already a standard practice. Depositing metallic NiTi shape memory alloy coatings may be a viable alternative for hard ceramic coatings. NiTi coatings offer additional benefits originating from unique functional thermomechanical properties. However, fabrication of thick NiTi coatings turned out to be difficult. Standard electroplating and laser cladding methods are not suitable for NiTi the most widely used plasma spray methods tend to produce chemically inhomogeneous coatings that do not transform martensitically, cold sprayed NiTi coatings suffer from poor adhesion to the substrates. In this work we report on first ever successful fabrication of thick NiTi coatings (100-300 um) that display functional thermomechanical properties and simultaneously show very good adherence to the substrate. We used high velocity air fuel thermal spray method to fabricate NiTi coatings deposited on mild steel using four different sets of processing parameters. Chemical composition, porosity, microstructure, phase transformation and functional thermomechanical properties of the NiTi coatings were evaluated. Although the coatings contain inhomogeneous microstructure, voids, oxide particles, high density of dislocation defects and internal stress, they undergo martensitic transformation upon cooling and or mechanical loading. As sprayed NiTi coatings need to be annealed to display functional thermomechanical properties. Despite their limited tensile strength, the coatings displayed thermal actuation in 3 point bending tests and shape memory effects in nanoindentation and scratch tests.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 1 minor

Summary. The manuscript reports the first successful fabrication of thick (100-300 μm) NiTi shape memory alloy coatings on mild steel substrates via high-velocity air-fuel (HVAF) thermal spraying using four processing parameter sets. The coatings are characterized for composition, porosity, microstructure, phase transformation, and thermomechanical behavior; despite inhomogeneities, voids, oxide particles, high dislocation density, and internal stresses, they are claimed to undergo martensitic transformation upon cooling or loading and to exhibit functional properties (thermal actuation in 3-point bending, shape memory in nanoindentation and scratch tests) after annealing, with good substrate adherence.

Significance. If the quantitative evidence supports reversible, functional transformation despite defects, the work would represent a meaningful advance in producing adherent, thick SMA coatings that combine wear/corrosion resistance with thermomechanical functionality, addressing limitations of prior methods like plasma spray and cold spray. The experimental use of HVAF and multi-test validation of actuation are strengths.

major comments (2)
  1. [Abstract] Abstract: the central claim that the coatings 'display functional thermomechanical properties' and 'undergo martensitic transformation' despite listed defects is load-bearing but unsupported by any reported transformation temperatures, hysteresis widths, recovered strain fractions, or cycle stability metrics; without these, it is impossible to confirm reversibility or practical functionality.
  2. [Abstract] Abstract: the assertion of 'very good adherence to the substrate' and 'first ever successful fabrication' of thick coatings with preserved properties requires explicit quantitative adhesion data (e.g., bond strength values) and direct comparison to prior literature results on NiTi coating adhesion and transformation; these are essential to substantiate the novelty and success claims.
minor comments (1)
  1. [Abstract] Abstract contains minor grammatical issues ('and or' should be 'and/or'; 'need to be annealed' is awkward).

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive comments on our manuscript. We address each major comment point-by-point below, indicating planned revisions where they strengthen the presentation of our results without overstating the data.

read point-by-point responses

  1. Referee: [Abstract] Abstract: the central claim that the coatings 'display functional thermomechanical properties' and 'undergo martensitic transformation' despite listed defects is load-bearing but unsupported by any reported transformation temperatures, hysteresis widths, recovered strain fractions, or cycle stability metrics; without these, it is impossible to confirm reversibility or practical functionality.

    Authors: The manuscript reports DSC curves with explicit martensite start/finish and austenite start/finish temperatures plus hysteresis widths, and 3-point bending data with recovered strain values after heating. These appear in the results section and figures. We agree the abstract would be stronger with these numbers included and will revise it to report the key transformation temperatures, hysteresis, and approximate recovered strain fractions. Cycle stability was not measured (only single thermal or loading cycles were performed), so that metric cannot be added. revision: yes

  2. Referee: [Abstract] Abstract: the assertion of 'very good adherence to the substrate' and 'first ever successful fabrication' of thick coatings with preserved properties requires explicit quantitative adhesion data (e.g., bond strength values) and direct comparison to prior literature results on NiTi coating adhesion and transformation; these are essential to substantiate the novelty and success claims.

    Authors: We will expand the introduction and discussion with direct comparisons to published plasma-sprayed and cold-sprayed NiTi coatings, citing their reported adhesion issues and lack of transformation. For adherence, the current evidence is qualitative (cross-section SEM showing continuous interface without delamination after spraying, annealing, and testing). Quantitative bond-strength values were not measured in this study, so we cannot supply them; we will revise the abstract wording from 'very good adherence' to 'good adherence evidenced by...' to reflect the available data. revision: partial

standing simulated objections not resolved
  • Quantitative bond strength (adhesion) values, as these measurements were not performed.

Circularity Check

0 steps flagged

No circularity: purely experimental report with direct measurements only

full rationale

The paper reports experimental fabrication of NiTi coatings via HVAF spraying and subsequent characterization of microstructure, composition, porosity, phase transformation, and thermomechanical behavior. No equations, fitted parameters, predictions, ansatzes, or derivation chains appear in the provided text. All statements reduce to direct experimental observations (e.g., SEM, DSC, nanoindentation, bending tests) rather than any self-referential modeling or self-citation load-bearing steps. The central claim of functional properties despite defects is supported by the reported measurements themselves, with no reduction to prior inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental materials processing paper; no mathematical model, free parameters, or new postulated entities are introduced.

pith-pipeline@v0.9.1-grok · 5868 in / 996 out tokens · 23992 ms · 2026-07-03T09:45:07.841057+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

32 extracted references · 26 canonical work pages

  1. [1]

    M.; Leary, M.; Subic, A.; Gibson, M

    Jani, J. M.; Leary, M.; Subic, A.; Gibson, M. A. A review of shape memory alloy research, applications and opportunities. Materials & Design 2014, 56, 1078-1113. https://doi.org/10.1016/j.matdes.2013.11.084

    work page doi:10.1016/j.matdes.2013.11.084 2014

  2. [2]

    Azubuike Amadi, Mysara Mohyaldinn, Syahrir Ridha, Victor Ola, Advancing engineering frontiers with NiTi shape memory alloys: A multifaceted review of properties, fabrication, and application potentials, Journal of Alloys and Compounds 2024, 976, 173227, ISSN 0925-8388, https://doi.org/10.1016/j.jallcom.2023.173227

    work page doi:10.1016/j.jallcom.2023.173227 2024

  3. [3]

    Hongjie Jiang, Shanshan Cao, Changbo Ke, Xiao Ma, Xinping Zhang, Fine-Grained Bulk NiTi Shape Memory Alloy Fabricated by Rapid Solidification Process and Its Mechanical Properties and Damping Performance, Journal of Materials Science & Technology 2013, 29, 855 -862, ISSN 1005 -0302, https://doi.org/10.1016/j.jmst.2013.05.007

    work page doi:10.1016/j.jmst.2013.05.007 2013

  4. [4]

    Net-Shape NiTi Shape Memory Alloy by Spark plasma Sintering Method

    Samal, S.; Molnárová, O.; Průša, F.; Kopeček, J.; Heller, L.; Šittner, P.; M.; Abate, L.; Blanco, I. Net-Shape NiTi Shape Memory Alloy by Spark plasma Sintering Method. Appl. Sci. 2021, 11, 1802. https://doi.org/10.3390/app11041802

    work page doi:10.3390/app11041802 2021

  5. [5]

    Bermingham, M.J., McDonald, S.D., StJohn, D.H. et al. Segregation and grain refinement in cast titanium alloys. Journal of Materials Research 2009, 24, 1529–1535. https://doi.org/10.1557/jmr.2009.0173

    work page doi:10.1557/jmr.2009.0173 2009

  6. [6]

    Samal, Sneha., Sulovský, Mikulas, Kopeček, Jaromír., Šittner, Petr., Deposition and characterization of plasma sprayed NiTi powder on stainless steel and graphite substrate, Materials Chemistry and Physics 2023, 307, 128146, ISSN 0254-0584, https://doi.org/10.1016/j.matchemphys.2023.128146

    work page doi:10.1016/j.matchemphys.2023.128146 2023

  7. [7]

    Bram, M., Ahmad-Khanlou, A., Heckmann, A., Fuchs, B., Buchkremer, H.P., Stöver, D., Powder metallurgical fabrication processes for NiTi shape memory alloy parts, Materials Science and Engineering: A 2002, 337, 254- 263, ISSN 0921-5093, https://doi.org/10.1016/S0921-5093(02)00028-X

    work page doi:10.1016/s0921-5093(02)00028-x 2002

  8. [8]

    Dong-Dong Zhuang, Shu-Hao Zhang, Hai-Xia Liu, Jie Chen, Cavitation erosion behavior and anti -cavitation erosion mechanism of NiTi alloys impacted by water jet, Wear 2023, 518 –519, ISSN 0043 -1648, https://doi.org/10.1016/j.wear.2023.204631

    work page doi:10.1016/j.wear.2023.204631 2023

  9. [9]

    Tomáštík, L

    Samal, S., Cibulková, J., Čtvrtlík, R., J. Tomáštík, L. Václavek, J. Kopeček, P. Šittner, Tribological behavior of NiTi alloy produced by spark plasma sintering method, Coatings, 2021, 11, 1246

    2021

  10. [10]

    Journal of Material Engginering and Performance 2019, 28, 6033–6041

    Cameron, N., Farhat, Z., Investigation into the Wear and Dent Resistance of Ni/Ti Nanolaminates and Superelastic NiTi Coating. Journal of Material Engginering and Performance 2019, 28, 6033–6041. https://doi.org/10.1007/s11665-019-04377-5

    work page doi:10.1007/s11665-019-04377-5 2019

  11. [11]

    Shugo,Y.; Hanada, S.; Honma, T.; Bull. Res. Inst. Min. Dress. Metall. Tohoku. Univ., 41 1985, 35

    1985

  12. [12]

    Bram, M., Ahmad-Khanlou, A., Buchkremer, H.P., Stöver, D., Vacuum plasma spraying of NiTi protection layers, Materials Letters 2002, 57, 647-651

    2002

  13. [13]

    Fabrication of Thermal Plasma Sprayed NiTi Coatings Possessing Functional Properties

    Samal, S.; Tyc, O.; Cizek, J.; Klecka, J.; Lukáč, F.; Molnárová, O.; de Prado, E.; Weiss, Z.; Kopeček, J.; Heller, L.; et al. Fabrication of Thermal Plasma Sprayed NiTi Coatings Possessing Functional Properties. Coatings 2021, 11, 610. https://doi.org/10.3390/coatings11050610 26

    work page doi:10.3390/coatings11050610 2021

  14. [14]

    Samal, S., Zeman, J., Kopeček, J., Šittner, P., Thermal plasma spraying of NiTi powder for thick coating of shape memory alloy, Journal of Alloys and Compounds 2024, 984, 173962, ISSN 0925-8388, https://doi.org/10.1016/j.jallcom.2024.173962

    work page doi:10.1016/j.jallcom.2024.173962 2024

  15. [15]

    Samal, S., Thermal plasma technology: The prospective future in material processing, Journal of Cleaner Production 2017, 142, 3131-3150

    2017

  16. [16]

    & Gill, H.S

    Kumar, K., Kumar, S. & Gill, H.S. High -Temperature Erosion–Corrosion Performance of Plasma and High - Velocity Oxy-Fuel Sprayed TiC and TiN Enriched Ni -20Cr Coatings in an Actual Boiler Environment. J. of Materi Eng and Perform 2025, 34, 17900–17914. https://doi.org/10.1007/s11665-024-10465-y

    work page doi:10.1007/s11665-024-10465-y 2025

  17. [17]

    Samal, Sneha., Vokoun, David., Stachiv, Ivo., Šittner, Petr., Simulation of thermal stress predictions for NiTi coating on a stainless-steel substrate using thermal plasma spraying, Ceramics International 2025, 51, 12337- 12345, ISSN 0272-8842, https://doi.org/10.1016/j.ceramint.2024.10.328

    work page doi:10.1016/j.ceramint.2024.10.328 2025

  18. [18]

    Preparation and Characterization of Multilayer NiTi Coatings by a Thermal Plasma Process

    Samal, S.; Zeman, J.; Habr, S.; Pacherová, O.; Kopeček, J.; Šittner, P. Preparation and Characterization of Multilayer NiTi Coatings by a Thermal Plasma Process. Materials 2024, 17, 694. https://doi.org/10.3390/ma17030694

    work page doi:10.3390/ma17030694 2024

  19. [19]

    Interfacial Adhesion of Thick NiTi Coating on Substrate Stainless Steel

    Samal, S.; Kopeček, J.; Šittner, P. Interfacial Adhesion of Thick NiTi Coating on Substrate Stainless Steel. Materials 2022, 15, 8598. https://doi.org/10.3390/ma15238598

    work page doi:10.3390/ma15238598 2022

  20. [20]

    The Microstructure, Hardness, Phase Transformation and Mechanical Properties of a NiTi Coating Applied to Graphite Substrate via a Plasma Spraying Process

    Samal, S.; Zeman, J.; Kopeček, J.; Šittner, P. The Microstructure, Hardness, Phase Transformation and Mechanical Properties of a NiTi Coating Applied to Graphite Substrate via a Plasma Spraying Process. Coatings 2023, 13, 1174. https://doi.org/10.3390/coatings13071174

    work page doi:10.3390/coatings13071174 2023

  21. [21]

    Cano, Coatings, Surface Modifications, Spray Techniques (Cold Spray, HVOF/HV AF), Editor(s): Francisca G

    Vicente Albaladejo-Fuentes, Ana Maria Martos, Alessio Silvello, Sergi Dosta, Javier Sanchez, Irene G. Cano, Coatings, Surface Modifications, Spray Techniques (Cold Spray, HVOF/HV AF), Editor(s): Francisca G. Caballero, Encyclopedia of Materials: Metals and Alloys, Elsevier, 2022, 451 -464, ISBN 9780128197332, https://doi.org/10.1016/B978-0-12-819726-4.00124-1

    work page doi:10.1016/b978-0-12-819726-4.00124-1 2022

  22. [22]

    Verdian, M.M.; Raeissi, K.; Salehi, M.; Corrosion performance of HVOF and APS thermally sprayed NiTi intermetallic coatings in 3.5% NaCl solution, Corrosion Science 2010, 52, 1052 -1059, ISSN 0010 -938X, https://doi.org/10.1016/j.corsci.2009.11.034

    work page doi:10.1016/j.corsci.2009.11.034 2010

  23. [23]

    Verdian, M.M.; Raeissi, K.; Salehi, M.; Electrochemical impedance spectroscopy of HVOF -sprayed NiTi intermetallic coatings deposited on AISI 1045 steel, Journal of Alloys and Compounds 2021, 507, 42-46, ISSN 0925-8388, https://doi.org/10.1016/j.jallcom.2010.07.132

    work page doi:10.1016/j.jallcom.2010.07.132 2021

  24. [24]

    https://www.uniquecoat.com/legacy-hvaf-guns

  25. [25]

    Rodríguez, Carvajal J.; Physica B 192 55(1993)

    1993

  26. [26]

    Mohamed E. Mitwally, Mahmoud Farag, Effect of cold work and annealing on the structure and characteristics of NiTi alloy, Materials Science and Engineering: A 2009, 519, 155 -166, ISSN 0921 -5093, https://doi.org/10.1016/j.msea.2009.04.057

    work page doi:10.1016/j.msea.2009.04.057 2009

  27. [27]

    Hongliang Yang, Jian Chen, Yahua Li, Xiaobin Shi, Yongtao Li, Junsong Zhang, Effect of cold work on martensitic transformation of Ni38Ti37V25 alloy reinforced by V nanowires, Journal of Alloys and Compounds, V olume 815, 2020, 152489, ISSN 0925-8388, https://doi.org/10.1016/j.jallcom.2019.152489

    work page doi:10.1016/j.jallcom.2019.152489 2020

  28. [28]

    Fatigue Resistance Improvement in Cold-Drawn NiTi Wires Treated with ALD: A Preliminary Investigation

    V okoun, D.; Tyc, O.; Samal, S.M.; Stachiv, I.; Yu, Y .; Kei, C. Fatigue Resistance Improvement in Cold-Drawn NiTi Wires Treated with ALD: A Preliminary Investigation. Applied Science, 2025, 15, 1823. https://doi.org/10.3390/app15041823 27

    work page doi:10.3390/app15041823 2025

  29. [29]

    Thangamani, G., Tamang, S.K., Patel, M.S. et al. Post-processing treatment of Wire Arc Additive Manufactured NiTi shape memory alloy using laser shock peening process: a study on tensile behavior and fractography analysis. Int J Adv Manuf Technol 2025, 136, 3315–3327 https://doi.org/10.1007/s00170-025-15058-0

    work page doi:10.1007/s00170-025-15058-0 2025

  30. [30]

    Václavek, Lukáš; Samal, Sneha; Tomáštík, Jan; Molnárová, Orsolya; Šittner, Petr; Multi-cycle nanoindentation, scratch and wear test on super -elastic (SE) and shape memory (SM) NiTi alloy, Results in Surfaces and Interfaces 2026, 23, 100768, ISSN 2666-8459, https://doi.org/10.1016/j.rsurfi.2026.100768

    work page doi:10.1016/j.rsurfi.2026.100768 2026

  31. [31]

    Samal, S., Tomáštík, J., Čtvrtlík, R., Václavek, L., Molnárová, O., Šittner, P., Surface Deformation recovery by thermal annealing of thermal plasma sprayed shape memory NiTi Alloys, Coatings 2023, 13 , 433, 10.3390/coatings13020433

    work page doi:10.3390/coatings13020433 2023

  32. [32]

    Samal, Sneha., Tomáštík, Jan., Václavek, Lukáš., Chandra, Mohit., Kopeček, Jaromír., Stachiv, Ivo., Šittner, Petr., Recovery of deformation surface of superelastic and shape memory NiTi alloy, Applied Surface Science Advances 2025, 25, 100684, ISSN 2666-5239, https://doi.org/10.1016/j.apsadv.2024.100684

    work page doi:10.1016/j.apsadv.2024.100684 2025