Ultrasonic Modification of Wood Surface: Study of Macro and Micro Properties after Long-Term Storage

In this paper, the stability of the results of ultrasonic wood surface modification after long-term storage, including macroscopic properties and microstructure of specimens, was investigated. Specimens of aspen wood (Populus tremula) were processed by the developed ultrasonic method of wood surface modification in three different treatment modes and the surface hardness of the specimens was evaluated after processing and after storing the specimens for more than 5 years since long-term stability is an important factor for the use of ultrasonically modified sawn timber as construction and finishing materials. The obtained results of surface hardness measurements by the Leeb method showed that the decrease in hardness after long-term storage is approximately 6.6% for the lowest degree of treatment and approximately 3.4% and 2.4% for medium and high degrees of treatment, taking into account the fact of the average increase in surface hardness approximately 2–4 times, this decrease is insignificant. The internal structure of the specimens after storage was studied by scanning electron microscope (SEM), and deformations of the wood surface layer without damage or rupture were analyzed. The derived stable results confirm the potential of the ultrasonic method for wood surface modification.