Positron annihilation spectroscopy studies of defects induced by surface mechanical treatments
News, 24 August 2022
The knowledge of the impact of the given surface treatment process is important for preparing better-quality parts of machines, dental prosthetics, and much more. The influence of treatment parameters on the defect structure was investigated in the cycle of studies at JINR. Data has been obtained on which defects form under the surface in the so-called subsurface zone during blasting and sliding. Ultimately, this data can affect the corrosion and radiation resistance of the material. The research was carried out by positron annihilation spectroscopy in cooperation with scientific organizations in Russia and Poland.
Positron annihilation spectroscopy is a highly sensitive non-destructive method for the study of structural defects in solids, including nanomaterials. In this method, high-energy positrons are emitted by a radioactive source and then sent into matter, where the positron, meeting its antiparticle electron, annihilates with the emission of gamma quanta, which are detected in a gamma quanta detector. Material defects are identified by changes in positron annihilation rate.
For the cycle of works “Positron annihilation spectroscopy studies of defects induced by surface mechanical treatments” a group of authors consisting of Krzysztof Siemek (LNP JINR; INP PAS, Cracow, Poland), Paweł Horodek (LNP JINR; INP PAS, Cracow, Poland), Jerzy Dryzek (INP PAS, Cracow, Poland), Marat Kanalbekovich Eseev (NArFU, Arkhangelsk), Mirosław Wróbel (AGH University, Cracow, Poland) was awarded the second JINR Prize in 2021 in the category “Scientific-methodical and scientific-technical works”. The works of the cycle were carried out at the Dzhelepov Laboratory of Nuclear Problems JINR.
Krzysztof Siemek, one of the researchers, a recent head of the Electronic Cooling Sector at DLNP JINR, spoke about the research.
“Mechanical surface treatment, such as blasting, cutting, sliding and machining, introduces changes in the microstructure on the surface but also below it. The subsurface zone is characterised by different physical and mechanical properties other than the properties of the internal volume of the material. The surface treatment process leads to the creation of several kinds of structural defects with unknown distributions and occupied depths. Their occurrence is revealed in changes in strength, ductility, and microhardness. Corrosion can also be connected with defects. Moreover, faster wear of materials starts at the atomic level and can affect the lifetime of the material,” he said and added that the studies of the subsurface zone are usually performed using conventional engineering methods, such as microhardness tests, and other analytical techniques like electron microscopy or X-ray diffraction. However, plastic deformations occurring during treatment generate structural defects, which are largely undetected by the standard methods. Vacancies and their clusters, are observed using positron annihilation spectroscopy.
This type of processing involves bombarding the surface of the material with small abrasive particles. This process is used for cleaning surfaces of metallic products but also for their decoration and improving their texture. Blasting can be characterized by several process parameters, such as size or type of abrasive particles, angle, pressure, and process duration. Changes generated on and below the surface are strongly dependent on these features. “Every single abrasive particle can be treated as a bullet launched on the surface. The impact of the hit depends on the particle mass and velocity as well as the direction of the collision angle. Bigger particles and higher compression of air used to propel the blasting material cause higher deformations of the subsurface zone. Small collision angles generate a smaller concentration of defects beneath the blasted surface; however, strong surface erosion was also observed in this case. The studies have proven that erosion increases for small sandblasting angles,” explained Krzysztof Siemek
When two bodies are in a sliding contact, the load at the surface is supported by asperities, which undergo elastic or plastic deformation as soon as they are in contact. High stress concentration in these regions can lead to damage and thus also to crack initiation. The asperity region can be the source of dislocations (crystalline defects) that lead to stress concentration in the subsurface zone. It is not excluded that the impacts between asperities cause deformation of bodies with worn surface observed at a depth of hundreds of micrometers from the surface. The size of asperities is on the order of a micrometer or less. Defects concentration decreases with increasing depth from the worn surface. The mechanism of subsurface zone generation is still poorly understood. The collection of new experimental data for various metals and alloys can help get to know it.
The oxide layer present on the metal surface is of great importance for prosthetics, influencing oxidation resistance. Commonly used tooth implants have a two-layer structure: the outer layer, which ensures aesthetics, is built from weak ceramics or composite connected with a strong supporting core made of ceramics or metal alloys on the metal–veneer bond strength Base metal alloys are formed by the elements that are passivated. The procedure is for coating the metal surface with a thin, corrosion-resistant layer. Passivation is carried out mainly with chromium. One of the challenges in Cr-containing alloys is controlling the excessive formation of chromium oxide that results in a lower bond strength between base metals and a veneer.
Oxidation products and organic contaminants are removed mainly by blasting. After sandblasting, some abrasive particles remain adhered to the blasted surface and can positively influence adhesion properties of a material. The researchers examined the effect of blasting parameters and material strength on the retention of aluminium oxide and found that other surface treatments also influence the oxide structure in the surface and affect other material properties. The oxidation behavior of Ni-based superalloys changes as a result of grinding and polishing. On the polished surface, a multilayered oxide scale consisting of NiO, Cr2O3 was found, and in the internal zone, it was of Al2O3, while on the ground surface, mainly the protective Al2O3 formation was observed. In the case of the studied Ni-based superalloys, polishing showed a negative effect in terms of the high-temperature oxidation behavior. It was shown that, by the simple mechanical surface preparation method, the alloy could be moved from the region of alumina formation into chromia forming alloys.
“Surface treatment can lead to nanocrystallization. The treated material is subsequently annealed, and the produced nanosize grains improve surface properties. Dislocations are the basis of this process. It has been proven that the grain boundaries in the irradiated polycrystalline material can influence defect concentrations because they act as sinks where created defects disappear. Consequently, this can be considered another area of application for surface treatment techniques,” Krzysztof Siemek concluded.
For example, to avoid the irradiation damages created by long-time neutron exposition, researchers suggested reducing the grain size and using heavy ions to treat the surface instead of neutrons. Nuclear materials have to withstand neutron irradiation for many years, which causes damage to them. The implantation range of heavy ions is limited to a depth of several μm. The typical nanostructured area obtained by mechanical surface treatment significantly exceeds the range of heavy ions. It means that fundamental microstructure changes caused by neutron degradation can be performed in a thin nanostructured surface prepared by easily applicable mechanical methods and heavy-ion irradiation.
Mechanical surface treatments allow one to get grain sizes in the surface region in wide ranges up to sizes even not available for typical methods exploiting high pressure, such as high-pressure torsion, hydrostatic extrusion, equal channel angular pressing, etc. This gives the possibility of studying irradiation degradation of nanostructured metals and application of surface treatments as a finishing method for enhanced surface radiation resistance.
- different types of the mechanical surface treatment can be used as an effective tool for surface modification. They cause plastic deformations and introduce different types of defects below the subsurface zone. These defects depend on the material and mainly consist of a mixture of dislocations and monovacancies or vacancy clusters. [1-15]
- The defect concentration decreases with the depth and depends on treatment parameters and material properties, e.g., hardness. The total thickness of the subsurface zone:
- for blasting, increases with pressure, depends of the size of abradant and the impact angle; [1-9]
- for sliding, increases with the load. [11-14]
- The roughness of the sandblasted surface increases with the blasting impact angle, size of abradant, pressure. The impact of the blasting time is imperceptible. [1-9]
- The nitrogen atmosphere during annealing provides insufficient protection from the formation of layers below the stainless-steel surface. Blasting erodes these layers. 
- The erosion process during the 60-s sandblasting under 0.1 MPa pressure cannot completely remove metal oxides from the dental alloys, and higher compression of air 0.4 MPa is needed to clean the surface. 
- The deposition of alumina blasting particles depends on material hardness. A shallow deposition is noted for hard metals, e.g., dental alloys, and deep Al2O3 retention in soft metals, e.g., copper. [4, 8]
- The cavitation erosion process can be monitored using positron annihilation spectroscopy methods. The incubation period of the cavitation process can be used as a non-polluting surface modification technique that replaces blasting. 
- The surface preparation influences the oxidation behavior of Ni-based alloys. In the case of Ni-based alloys, polishing showed a negative effect in terms of the high-temperature oxidation behavior. 
- The mechanical surface treatment can lead to nanocrystallization. Nanostructuration of Ti obtained using blasting can reduce the amount of irradiation vacancy clusters almost by a factor of two compared to coarse-grain samples. The disappearance of vacancy clusters after annealing at 200 ◦C indicates the promising self-healing property of Ti. 
- The defect profile in the subsurface zone of pure silver exposed to dry sliding contains monovacancies up to the depth of 110 μm; Beyond, vacancies associated with dislocations and undamaged regions were found. The total depth of the subsurface zone is more than 300 μm, and it is hardly affected by the applied load. 
- The subsurface zone in pure niobium samples exposed to dry sliding revealed the defect depth profile extended in the range from 70 to 140 μm depending on the applied load from 5 to 50 N. No vacancy clusters were noted. 
- For pure Zr exposed to dry sliding, complex defect distributions are observed. Close to the worn surface only dislocations are present; at a certain depth, vacancy clusters occur, and their size decreases with increasing depth. Farther from the worn surface, dislocations are present again. [11, 12]
- P. Horodek, J. Dryzek, A. G. Kobets, M. Kulik, V. I. Lokhmatov, I. N. Meshkov, O. S. Orlov, V. Pavlov, A. Yu. Rudakov, A. A. Sidorin, K. Siemek, S. L. Yakovenko, Slow positron beam studies of the stainless-steel surface exposed to sandblasting Acta Physica Polonica A 125 (2014) 714, doi:10.12693/APhysPolA.125.714, Impact Factor: 0.643;
- P. Horodek, M. K. Eseev, A. G. Kobets, Studies of stainless steel exposed to sandblasting, Nukleonika, 60 (2015) 721, doi:10.1515/nuka-2015-0129, Impact Factor: 0.941;
- P. Horodek, K. Siemek, J. Dryzek, A. G. Kobets, M. Wróbel, Positron annihilation and complementary studies of stainless steel exposed to sandblasting at different angles, Tribol. Lett. 64 (2017) 30; doi:10.1007/s11249-017-0813-0, Impact Factor: 3.193;
- P. Horodek, K. Siemek, J. Dryzek, M. Wróbel, Positron Annihilation and Complementary Studies of Copper Sandblasted with Alumina Particles at Different Pressures, Materials 10 (2017) 1343; doi:10.3390/ma10121343, Impact Factor: 3.623;
- S. Kurdyumov, K. Siemek, P. Horodek, Positron annihilation spectroscopy studies of bronze exposed to sandblasting at different pressure, J. Phys.: Conf. Ser. 791 (2017) 012029 doi:10.1088/1742-6596/929/1/012029, Impact Score: 0.55;
- K. Skowron, K. Siemek, Positron annihilation spectroscopy studies of sandblasted copper, Acta Physica Polonica B Proceedings Supplement 11 (2018) 4 doi:10.5506/APhysPolBSupp.11.795, Impact Score: 0.38;
- P. Horodek, K. Siemek, J. Dryzek, M. Wróbel, Impact of abradant size on damaged zone of 304 AISI steel characterized by positron spectroscopy, Metallurgical and Materials Transacions A 50 (2019) 1502, doi:10.1007/s11661-018-5067-4, Impact Factor: 2.625;
- K. Siemek, M. Kulik, M. K. Eseev, A. G. Kobets, M. Wróbel, O. S. Orlov, A. A. Sidorin, Surface and subsurface defects studies of dental alloys exposed to sandblasting, Acta Metallurgica Sinica (English Letters) 32 (2019) 1181, doi:10.1007/s40195-019-00884-5, Impact Factor: 2.090;
- K. Siemek, P. Horodek, V. A. Skuratov, J. Waliszewski, A. Sohatsky, Positron annihilation studies of irradiation induced defects in nanostructured titanium, Vacuum 190 (2021) 110282, doi:10.1016/j.vacuum.2021.110282, Impact Factor: 2.906;
- K. Siemek, M. K. Eseev, P. Horodek, A. G. Kobets, I. V. Kuziv, Defects studies of nickel aluminum bronze subjected to cavitation, Applied Surface Science 546 (2021) 149107, doi:10.1016/j.apsusc.2021.149107, Impact Factor: 6.182;
- J. Dryzek, K. Siemek, Formation of subsurface zone induced by sliding wear in zirconium studied by positron lifetime spectroscopy, Tribol. Lett. 64 (2016) 15; doi:10.1007/s11249-016-0747-y, Impact Factor: 3.193;
- J. Dryzek, P. Horodek, Slow positron beam studies of zirconium exposed to dry sliding, Journal of Physics: Conference Series, 791 (2017) 012020, Impact Score: 0.55;
- J. Dryzek, P. Horodek, Positron Annihilation Studies of the Near-Surface Regions of Niobium before and after Wear Treatment, Tribology Letters, 65 (2017) 117, doi: 10.1007/s11249-017-0902-0, Impact Factor: 3.193;
- J. Dryzek, K. Siemek, Positron Annihilation Studies of Subsurface Zone Created during Friction in Pure Silver, Tribology Transactions 62 (2019) 658, doi:10.1080/10402004.2019.1600769, Impact Factor: 1.511;
- W. Nowak, K. Siemek, K. Ochał, B. Kościelniak, B. Wierzba, Consequences of different mechanical surface preparation of Ni-base alloys during high temperature exposure, Materials 13 (2020) 3529, doi:10.3390/ma13163529, Impact Factor: 3.623.