Heat-treated wood (W HT) relatively enhanced dimensional stability, biological resistance, and aesthetic characteristics. HT has been employed at various temperatures (160–260 ☌) and various media reactions such as vacuum, nitrogen, steam, or oil. Recently, researchers focused on the combined effects of heat treatment (HT) and wood densification on the assumption that the post-thermal treatment could release the inner stresses and increase the stability of the compressed wood. (2018) successfully created a high-performance structural compressed wood via partial removal of lignin and hemicellulose and hot-pressing into highly aligned cellulose nanofibers, resulting in specific strength higher than that of most structural metals and alloys. Densification is proven to improve mechanical properties such as modulus of elasticity (MOE), surface hardness, and mechanical strength of wood, especially for low-density species. Densification has been reported for over a century as an efficient method for enhancing wood mechanical properties by reducing porosity and increasing density. Nevertheless, the drawback of using plantation timbers is mostly related to their low density, mechanical strength, and dimensional stability. Since the conservation mandates for natural forests were improved in China, the demand for plantation trees such as poplar, eucalypt, and pine has increased, making them a reliable alternative for industrial timbers. It appears that this NP-involved thermal densification could be considered as an alternative approach to enhance both the compression stability and fire resistance of wood.

Moreover, surface hardness increased by 15.8%, and the recovery of surface hardness and thickness were 56.8% and 77.2% lower than that of simply densified wood. Fire retardancy was also enhanced the total heat release and CO generation values decreased by 21.9% and 68.4%, respectively, when compared with that of solely NP-treated wood.

Results demonstrate that the combined treatment hardened the surface of wood and reformed the interface combination of the NP with the wood cell wall, thus making the surface tissue more close-grained. This combined treated wood is investigated via vertical density profile (VDP), and the compression stability is revealed by both soaking test and cone analysis. To enhance compression stability and fire retardancy of densified wood, a new modification method i.e., combined nitrogen–phosphorus (NP) fire retardant pre-impregnation with surface thermo-mechanical densification is used to fabricate a certain thickness of functionalized surface layer on poplar.

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